Augment the category menu by system tags and already used categories.

This commit add all available "collaborative tags" and all already used
categories into option groups of the tags-menu of the side-bar editor.

This commit addresses and is a related to a couple of open issues:

nextcloud/calendar#3735 Calendar Categories: Propose Categories already used

- this should be fixed by this commit

nextcloud/calendar#1644 Add own categories, delete default ones

- this is partly fixed in the sense that collaboritive tags are now also
  proposed as calendar categories.
- still default categories cannot be deleted
- however, using option groups one at least has some sort of overview
  about the origin of the proposed category

nextcloud/server#29950 Save VEVENT CATEGORIES as vcategory

- this issue is totally "ignored" by this commit as the proposed
  solution there is not needed (the categories are already there in the
  oc_calendarobject_props table)
- that would have to be discussed there: but my impression that the
  tables and classed mentioned there are obsolete and no longer used.

Co-authored-by: Anna <anna@nextcloud.com>
Signed-off-by: Claus-Justus Heine <himself@claus-justus-heine.de>