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...

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.

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.

#635

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

#552

#610

#618