- Add cdk scrolling strategy to keyboard that brings overlays to
scrolling

Drop lists that allow only one element but are not used in cloze texts
display their elements right-aligned

- Add left and right padding for input field

- Set the duration of highlighting anchors to 15 seconds
- Disable simultaneous highlighting of anchors

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

Read-only math fields are displayed without borders

HTML labels can cause errors in the Test Center for units in XML format.

- Show the error message only after the math field has been touched
- Add styles for error message

Since buttons can now act as switches for text-element highlighting, it 
is no longer exclusively used for navigation purposes.

- Mainly highlight border and moving elements.
- Centerpiece is: line-height: 130%;
  Don't know why or how but it works okay.

- 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.

- Add own line height style to error message to exclude it
- Add missing semicolons to styles

Rules for when drop is allowed have been fixed. It is quite complicated 
and therefore a lot of explanatory comments are added.

- improved translation
- add example file

This reverts commit f90bcc5b
Target installation seems not to support this Verona Api version yet