- Basic implementation

Also restructure files containing interfaces.

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

- 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

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.

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.

- Remove global dialog style

New character: protected dash (which does not break lines)

#455

flex-start is the correct value for flex containers.