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

Update/translate package

- A suitable version was not available after update of Angular
- Reactivate test for valid pages

#499

- Background color must not be transparent for the stickied panel to 
overlay properly.

- Use TextImagePanel where possible

Toolbar is a single word, therefore any capitalized letters are errors. 
No need to sanitize, since this was broken anyway.

#384

#386

... instead if setting the value directly on the object. This way 
elements can react to ID changes. This is necessary for DropLists since 
a changed ID has to be reflected to child elements.

#445

#453

Less unneeded scrollbars and fixed height.

Th actual clickable area is displaced by that, which is hard to fix. So 
the worse layout is accepted for now.

Material update fix

The wasted vertical margins did look bad.

Several elements were broken after updating the Material library.

Add new component for combobuttons, which allow selecting and applying a 
value in one connected element.

Most importantly remove the standalone switch and let the service always 
send postMessage, even when there is no surrounding application 
listening. This helps testability and has no downsides.

Also:
- Make methods static where applicable
- Remove useless variables and methods
- Improve variable initialisation (to allow undefined)