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.