Use unit field of UnitService directly to detect changes.

No longer have unit as subject. Components can use the unit variable as 
is and still get updated.
This removes some listener stuff in conected components.

The problem being solved is that compound-sub-elements must not have
all the properties other elements have. For exampe positioning info.
Sub-Elements are positioned inline. To avoid having similar class
inheritance structures for positioned and inline elements, we use
interface. Positioning is done via interface and is therefore optional.

Those optional properties are kept in an object in a variable. This
allows to hide specifics from element classes and removes the need to
initialize all the fields in every class using the interface.
The elements mostly care about their own specific properties anyway.

The same technique is used for font and surface elements. Here we also
don't want to initialize all the styles in every class.

Changing values of element properties works like before. Just set the
property as if it would be a direkt prop of the element. The element
itself puts the value in the currect sub-object (positioningProps,
fontProps etc).

For reading the values there is a similar method, but it is not used
anywhere by now. Since the properties panel operates on the raw values
anyway (refer CombinedProperties) and for element components this would
mean calling a function in the template, which caused change detection
to run the function very often and is therefore bad practice.

Additional notes and refactorings:
- Restructure all files in common. UIElements in the same folder etc.
- Move all interfaces to the UIElement file. This avoids circular
  imports, which would be needed to allow PositionedElement to extend
  UIElement.
- Interface initializers also handle reading values in the old form.
- Add PositionedElement for use in canvas overlays. This is a UIElement
  with guaranteed postionProps.
- Don't export all Material packages from common, only the ones
  used in player and editor. The rest is only used in common and
  does not need to be exported.

This structure moves closer to the structure template the team decided
on. Components and services are kept in dedicated folders instead of
following the logical code structure. This is still not the case here as
the unit-view component is still nested, but it's closer.

This works similarly to the standard radio group, but has a fixed 
horizontal layout with images above the options.

Other components will follow.

The main components are right at top level and the directories below
mimic the data structure (unit->page->section).
The toolbar is only used in dev mode and does not belong directly to the
unit view.
The TextEditor is a complex component and deserves it's own directory
even though it is used within the unit-view. Might be refactored to its
own module later.

- The selection service only needs to handle element selection. For
pages and sections having the index is enough.
- Those indeces are simple fields instead of observables. This also
simplifies the components which don't have to get the value out of the
observables.
- Therefore the selection service does no longer need to have a direct
reference to the unit service and it's unit.
- Selection logic is not done by the section components themselves but
by the parent canvas component.
- The unit service can now manipulate pages and section by index in
addition to passing the necessary objects directly.

New elements are now put on the current(/newly created) page or section.

Remove useless methods and add comments.

This also removes all page selection logic from the selection service.

It is now always made the first when page and it is taken out of the the 
index counter. So other pages start with 1, when a page is set as always 
visible.

Also rework selected page index to be managed by the selection service.

There is a bug in Angular and as a result Material tab elements brake 
when the underlying array is changed. To circumvent that we temporarily 
remove the tab element and add it again, which should re-initialize it.

Unsubscribing, private methods ...

- Unit service now again takes the elements to be changed as parameter
instead of managing (and even knowing) selections.
- Selections are managed by the appropriate service
- Also using rxjs operators instead of serving observable state as
getter method.

- ... mostly for dynamic positioning
- Refactor reordering of sections. They can now be moved by small
buttons to the side, which appear when hovering. The section edit mode
is no longer available. -> KISS
- Removed all toolbars. The props can now be set via the props panel,
which now has sections for pages, sections and elements. This is to have
more vertical space available for editing the unit.
- Sections can now (only) be added via the toolbox panel, where the
second tab is now exclusively for sections. Future templates are
supposed to be whole sections, which can also be added from there.
- Refactor CSS layouting (flex and co) to have better handling of
vertically growing page (via sections)
- Some preparation for an upcoming overhaul of selection stuff
(again :))

- use takeuntil
- pass index values instead of observables to children

- Service is used by the editor exclusively so definition in
local app.module.

- Edit dialogs triggered via double click on drag overlay
- Unit Service now defines a default edit dialog for elements. For now
label or text if available.

One page can have this setting and is supposed to be shown at all times 
on the left side.

This does not need to be an observable as it is used only internally and 
in 1 component, which needs the new state only after calling update 
functions. Those functions can return the new value directly instead of 
via observable. And for store internal use a simple variable is 
sufficient.

Now there is a button at the side that can be used to slide the panels 
in and out.

There it can be used by the player as well.