[Ticket: #636][Ticket: #633]

[Ticket: #630][Ticket: #631][Ticket: #632]

[Ticket: #615]

[Ticket: #626]

[Ticket: #623]

[Ticket: #624]

A number of bugs have been reported,
some of which this commit should fix.

[Ticket: #617][Ticket: #620][Ticket: #621]
[Ticket: #622]

During the last meeting, some problems were pointed out.
This commit should fix some of them.

With the new quill format, some cleanups and
unnecessary double checks are no longer required.
This leads to a much better performance.

[Ticket: #613]

After the marks have been added,
the only thing missing is Katex.
Katex must be implemented in a different way,
because the html data has to be enclosed
by a Quill container.