This feature is incomplete. Button is hidden until it's done.

Only works for a few commands (mainly page and section stuff).

- Adds new menu to page-bar, which shows issued change commands with a 
button to rollback the latest change.
- Added warning to the UI element that this is a experimental feature 
and can destroy units.
- Undo is done by having the normal function to apply the change to the 
unit-object and another to undo this change. The HistoryService manages 
the list and calling rollack (via UnitService).
- Further refactiring to the unit services.

- Encapsule into separate component
- No more pop-in-controls
- slicker ui layout
- More tooltip with explanations

#523
- Improve sanitizing of position margins

#511

- Add ReferenceManager to check if deleted elements are referenced 
anywhere in the unit. If so display a warning.
- Move page logic to UnitService

Remove 'any's be adding proper types and using 'never'.

Change translation key and meaning from 'typeAreaWidth' to
'sectionWidth'

Remove case distinction when labeling the page's margin width

The width of a page results from the value for the maximum width plus
twice the value for the spacing

- Now does unit (page) manipulation itself instead of delegating 
everything to the UnitService. Still need to call the UnitService to 
make it notify the API about the change to the unit.
- Also improve the hack for refreshing the tab view.

It did not do anyhting anyway. Is now integrated in unit view.

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.

Use unit field of UnitService directly to detect changes.

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.

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 simply was no change listener on the input.

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.

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.

- 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.

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 :))

Replace pageIndication with translate pipe