And avoid global styling.

This is rather hard to implement via HTML inline style attributes. So we 
do it the classic way.
Could be reviewed, when that feature is implemented in TipTap:
https://github.com/ueberdosis/tiptap/issues/1514.

They are now modified via menu at the section and no longer via props 
panel.

Use global style for droplist cursor.

- ... mostly for dynamic positioning
- Refactor reordering of sections. They can now be moved by small
buttons to the side, which appear when hovering. The section edit mode
is no longer available. -> KISS
- Removed all toolbars. The props can now be set via the props panel,
which now has sections for pages, sections and elements. This is to have
more vertical space available for editing the unit.
- Sections can now (only) be added via the toolbox panel, where the
second tab is now exclusively for sections. Future templates are
supposed to be whole sections, which can also be added from there.
- Refactor CSS layouting (flex and co) to have better handling of
vertically growing page (via sections)
- Some preparation for an upcoming overhaul of selection stuff
(again :))

Somewhat messy because of more nested droplists and a handler triggering 
every pixel. But the best I can do for now. May revisit later.

Stronger then warning.

The extra styling via panelClass needs CSS definition in local project, 
so can not be set by the Service itself but relies on local CSS rules.