#664

The background color is set via data-attribute so it can be turned off 
for the parent div but still used in the child (button).

For Table compatibility.

- Remove superfluous width and height. This is already done by parent 
elements in player and editor.
- Remove superfluous div and use host element instead.

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.

Stretch across the whole line.

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.

#662

Images were not showing at all.

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

The index was one off, when looking up the correct child element.