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Unsubscribing, private methods ...

Also some small improvements to variable naming and file structure.

#23

To communicate with the host application.

This allows the API service to send the unit definition to the host.

- 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.