#662

Images were not showing at all.

#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%;}'

- 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

#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

- Reformat styles

For Droplists this needs to be done when multiple items are allowed.

For ToggleButtons it needs to be set when vertical orientation is used.

It is then centered without caring about text alignment. With multiple 
item it would not be possible to align all of them anyway.

The native browser-drag-and-drop does not work in tablets. Therefore 
back to Material which works.
Features are moved over with the exception of replacing items.

- Copy-Lists now always allow putting items back.

- Renew example files
- Update versions and notes

- Add left and right padding for input field

- When the height of the child elements of a cloze element is increased,
they all now grow out of their center

- Extra span element around cloze child elements served no visible 
purpose. It also makes sense that an inline div is not inside of a span, 
which is meant for text.
- CSS rules were overengineered and in part shadowed (and thereby 
pointless) by element styles from the template.
This is also in preparation for the DropList styling overhaul.

- If this property is set, the input of more characters than specified
in maxLength is prevented
- It also prevents the insertion of text.

- Reduce redundant code
- Prepare implementation of input limitations

- Replace Material Droplist with native HTML events
- Remove simple-drop-list element; Cloze elements now use the normal 
DropList element
- Add example units

- overflow of lists is now hidden to prevent dropable elements from
being displayed outside the container
- placeholders are hidden with flex orientation