Used by reference and delete dialogs.

Separating code changes is rather hard right now, so there is a lot of stuff that is part of this commit that should not. Details below.

- All elements have static fields that holds their title and icon. This way dynamic lists can be built (for example for delete dialog) which show title and icon for all elements. ALso icons and names can be changed in one place.
- Template-Tab has been added. This should have templates to help creating groups of ui elements. This is work in progress and there are actually no templates yet. So this will change or even be removed in the future. This is mostly UI, the code is "TODO".
- Refactoring and improvement for delete and reference dialogs
- Delete logic refactored for elements, sections and pages. There are no longer different dialogs when references are found, but instead one dialog that can do both.

Visual overhaul and another way of inserting a section. The section code 
is saved to a global variable which can be used as well. Does obviously 
not work accress edit sessions or different units, therefore the old way 
is preserved.

#610

Explicitly define a label to be shown, since the shown label is done via 
innerHTML.

- Replace throwing Errors with InstantiationErors in unit, page and
  section in strictInstantiation mode

#523
- Improve sanitizing of position margins

In Player-context the constructors are to fail, when the given 
properties are not exactly as expected. Therefoore they are validated. 
When this validation fails, an environment variable is checked to 
determine if the element should be created anyway with defaults. This 
switch is off in player-context. But the Editor is not so strict and can 
create elements with incomplete parameters.

There is also a new VersionManager which checks given unit definitions 
for version-validity. Upon failing this check Player and Editor show 
error messages.
The Editor accepts a certain unit definition version, but shows a 
warning.

The Editor checks units for invalid references after loading. If any are 
found, they are removed and an info-panel is shown.

- Swap the position of dialog buttons

- Swap the position of dialog buttons

- Add translations for operators
- Improve and extend the evaluation of visibility rules

- Fix typo in css classname

- 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