Add 2 new menu buttons to the section menu.
One simply copies the selected section to the clipboard.
The other opens a dialog where a copied section can be pasted. The 
section element's IDs are checked if they are available and a warning is 
shown.

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.

Components with bigger scope are put to the component root, canvas, 
props panle and new-element-panel.

Change import paths of CommonModule files in editor and player

This is now done only via the cdkDropListGroup directive which collects 
child drop lists. There is actually no need to connect them manually.

This makes the value unselectable.

Now uses the implicit grid in auto mode.

This removes all the classes and goes back to simple interfaces for
unit, pages etc and all elements. This makes creating them easier and
cleaner.
Things like specific needs when setting element properties and most
importantly compatibility handling is done by a special service module,
that knows all the elements and how to repair old their potentially
outdated definitions.

This is done according to the Angular best practices and minimizes
namespace collisions when using other packages which might use the same
selectors.

Technically the selectors are not even becessary since we use an Angular
component factory to create element components but it is still useful
for the DOM element naming, for debugging purposes for example.

Also focus the height input field upon opening the menu.
For the layout menu this does not work as checkboxes don't play nicely 
with menus.

In fact having anything but button in the menu is against the Material 
spec. But i still think this is the best way to achieve the usability we 
want.

The dropListList generation is now done when things are changed via the 
section menu and no longer on element update via unit service.
Also remove some interfaces to avoid circular imports. The interfaces 
are so small and insignificant that their form might as well be 
specified at usage point.

This allows to select elements without clicking them on the canvas, as 
elements may be obstructed or not visible for some reason.

The selection logic is a bit complicated because usually only the 
overlay components are usd for selection which are also the place to 
apply any highlighting on. Here we have to find the component element 
reference from the section. Therefore the double looping 
'getElementComponent' method.

Also when an elements is selected it briefly gets z-index 100 so it is 
moved to the front and visible and manipulatable.

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.

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.

Variable naming issue.

When those switches are on the size values are ignored and calculated 
automatically instead.

The duplication is done by creating a new element (UnitUIElement)
for each element of the copied object. And then overwriting it with
the props of the old one, in addition to assigning a new ID.

The section menu (containing all former properties options and
reordering) is now shown on section hover.
- With the removal of the section edit mode the view-only elements
are no longer needed, as sections can no longer be dragged.
- The unit service now has a unified logic for moving items in an array,
being used for pages and sections. Although for pages there is an
additional regarding the always shown page.
The reorder function silently ignores nonsense reorders!
- The prompt dialog now takes the text as parameter

They are now modified via menu at the section and no longer via props 
panel.

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

Sections are always the same width as the page they are on.

When this is set the page will only be rendered to that width. Otherwise 
100% of the available space is used.

Now new input elements appear when more columns/rows are added, where 
value and unit can be changed separately.

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.

- Add (editable) rows to the grid
- Add drag and drop functionality to grid elements. The element is in 
'absolute' positioning mode while being dragged and snaps back to the 
grid when droppped.

Also refactor unit service to use more it's own selection variable 
instead of using passed references for manipulating objects.

- ... 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 :))