- 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.

Move `MatDialogModule` from editor to common
Add `AlertDialog` to player

Check whether `unitDefinition` or `unitStateDataType` are undefined is
done within the verify methods

* Define data version types data versions for `unitDefinition` and
`unitState` in meta tag of index.html
* Separate `MetaDataService` from `VeronaPostService`
* Extend `MetaDataService` to verify data versions of `unitDefinition` and
`unitState`

When elements are dynamically positioned there is no space below the elements

* Use `IntersectionObserver` to detect page position while scrolling

#14

- 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.

Now uses internal selected elements instead of element as parameter.

* Pages can be scrolled or tabbed
* Labels of pages can be shown and hidden
* Always visible pages can be left, right, top or bottom
* Layout of pages is now handled in `LayoutComponent` and removed from
`PlayerStateComponent`
* Due to the `ScrollIndexDirective`, pages with scrollbars have similar
interfaces as pages with tabs

Property to define the position of always visible page

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

Merge the `unitDefinition` with the `unitState.dataParts` to insert the
saved values

Required to decide whether the `section` or `element` should be inserted
or appended to its `formArray`

Required after changes in element components
(commit: 9e3c0613)

- ... 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 :))

Sections and elements can have this prop. Elements only need it to 
determine what properties to show in the props panel.
The others are basic props for grid layouting. Refer to the grid 
documentation.

This is in preparation for dynamic layouting.
The actual dimensions are set by the overlay component which differs but 
is existent for both editor and player.

* Remove workaround with `Promise.resolve().then(() => ...`
* Use `changeDetectorRef.detectChanges()` after updating the validators

Make the initialization consistent

Getter work like functions and are executed every time Angular change
detection runs.