SpellCheck: Fix markup rebasing when simple edits are done (one char added)

Summary:
Spellcheck called KoTextBlockData::rebaseMarkup() with char position
in *document* instead of position in *block*, which resulted in incorrect markup in
all but the first block.

Adding checks for combined add/remove which cannot be handled by KoTextBlockData::rebaseMarkup().
This means that dropcapped lines will not cause rebase to be done. Instead a full spellcheck
will be triggered for each change. Also markup cannot always keep up with changes to dropcapped lines,
resulting in incorrect/missing markup until next spellcheck is triggered.
Fixes should be done in KoTextLayoutArea, to avoid unneccessary changes to the document.

Reviewers: boemann

Tags: #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D7303