Alos small refactoring.

- Uses katex library
- TipTap extension similar to Cloze child elements. Only the rendering 
is different and done directly within the TipTap extension.

#647

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

#684

All created IDs are now always registered as well.

#681

#686

New section components no longer get their number on creation. This has 
the effect that the section gets a new number and is updated again with 
all others after that.
Numbering of all sections is triggered in one place (UnitService) and 
updates all existing sections.

This reverts commit 6ddfd7f7.

This makes no sense in expert mode either but is harder to solve. :)

This would alles changing things that are deliberately left out in the 
props panel.

Not above.

Also add controls to non-expert mode.