Just ignore the color in the component.

- to allow the display properties to be overwritten

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).

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 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

#618

#579

SInce this is a non-Material element the font is not automatically 
inherited. Needs to be done explicitly.

#574

#574

style used in other cloze children causes the error
':host {display:flex !important; width: 100%; height: 100%;}'

#558

#559

- Remove all Roboto files

Since there is just one font, this setting can be safely ignored. This 
is easier than sanitizing all elements.

#499
- Problem was due to changes in the latest minor material update

#531

#520

- crossOutChecked adds line-through styling if checked. This is used in 
Cloze texts to cross out words.
- This means Checkbox is no longer a PositionedElement and does not 
validate PositionProperties