Handle removals of corresponding opening and closing tags for tasks with and without trackChanges

Handle removals of tags as follows:

• User deletes or cuts contents between two tags completely: tags must not be removed.
  IP: = is ok with TrackChanges on and off; unless the content inbetween is the only content in the segment => fix
• User cuts one tag only (e.g. closing tag):
  • with TrackChanges: keep the other tag (e.g. opening tag) + paste this one tag without adding the partner tag *)
  • without TrackChanges: the same

• User deletes one tag only (e.g. closing tag):
  • with TrackChanges: delete the other tag, too (e.g. opening tag)
  • without TrackChanges: do not delete the other tag **)

Check in all relevant browsers!

*) after a 'cut' with a tag, multiple 'pastes' will insert the tag multiple times - this is ok; when the segment gets saved, the tag-checker will notice this

**) background: currently we don't realize that the tag has been deleted; however this could be implemented e.g. with DOM's MutationObserver (https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver)