- remove 'none'

Also add separate file to handle drop-logic.

#671

#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).

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

Stretch across the whole line.

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.

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

Only background color makes sense here.

Remove button is permanently visible and bith buttons are now in the 
lower left corner

#643

This is only used by the Editor. It was in common while the 
SanitizationService was used by the Player which then could display 
errors. Sanitization has been refactored and this is no longer needed.