For Table compatibility.

Dimensions are mandatory but with only height and width.

This change is for GeoGebra-Elements, which have mandatory width and 
height.

Table children do not have some properties, like position.

Javascript truthiness...

The selected section is replaced with the new section when empty. 
Otherwise the new section is appended below the selected section.
There is no way to influence this behaviour (contrary the 
section-insert-dialog).

This fixes Tables which houses elements without dimensions.

This way the props do not show in the prop-panel.

The default value overwrites the value set in the parent constructor 
(Javascript...). Therefore the value needs to be intialized explicitly 
after the parent constructor has run.

#659

Only background color makes sense here.

#669

The icon 'table' does look different for some reason. Taking the second 
best available icon.

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.

Only works for a few commands (mainly page and section stuff).

- Adds new menu to page-bar, which shows issued change commands with a 
button to rollback the latest change.
- Added warning to the UI element that this is a experimental feature 
and can destroy units.
- Undo is done by having the normal function to apply the change to the 
unit-object and another to undo this change. The HistoryService manages 
the list and calling rollack (via UnitService).
- Further refactiring to the unit services.

#635

Move element specific components to sub folder and just generally 
improve the file/component structure.

#552