- to prepare markable support for text as compound children

- 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

Also add separate file to handle drop-logic.

#671

All created IDs are now always registered as well.

#691

- Due to the width of the scrollbar, text areas change their inner width
under Chromium when the scrollbar is displayed. The use of offsetWidth
instead of contentRect ignores the presence of the scrollbar. The
calculation is therefore less accurate, but sufficient for estimating
the number of rows

#664

#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.