This fixes Tables which houses elements without dimensions.

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.

- and AnswerSchemeValue with VariableValue

- Method is already defined in superclass

#596

This holds variable names, taken from the GeoGebra applet, and uses 
those variable values in the value. This makes result analysis easier.

#511

GGB elements without toolbar aren't loaded, if this parameter is true

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.

- Model constructors (elements, unit, page, section) now have a 
dedicated parameter, which makes sure all properties are set and valid. 
This way the player can instantiate models without having to interpret 
the values. Any interpretation is done only by the editor, which now is 
solely responsible for creating defaults/blueprints for new elements. 
Models can no longer be created by incomplete paramaters.

- Also refactor Stylings to have sub groups for common styles

Also restructure files containing interfaces.

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

#384

Also set geometry element to always use fixed size, which is all that 
works anyway.

The unit tests errored out because of circular dependencies via the 
Elementfactory class. Therefore:
- No longer creates styling/position/player-Properties. This is done in 
the Element parent class.
- Move element creation from Section to the ElementFactory class 
instead.