Elements were instantiated a second time by the section constructor. Now 
the section is created without elements and the elements are added 
afterwards.

Alos small refactoring.

- to allow the display properties to be overwritten

- Replace foreach loops with map methods

- remove 'none'

- 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