- remove 'none'

Also refactor a lot of stuff around the page canvas and unit view.

#684

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