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

- Remove dynamicPositioning from PositionProperties. This is not 
knowledge that the element needs to know. In the few places it was used, 
it was fixed or solved by asking the containing section about which 
positioning is active.

- Fix and improve cloze child sizing. All can now have dynamic width and 
height.

- Fix empty lines in ClozeDocument rendering

- Editor SelectionService now knows if a cloze child is selected. This 
way the PropertiesPanel can show valid dimension inputs regardless of 
the underlying section.

Also restructure files containing interfaces.

#499

- Background color must not be transparent for the stickied panel to 
overlay properly.

- Use TextImagePanel where possible

- Reformat styles