Image annotations fixes

I think I accidentally included the changes from PR https://github.com/MarkUsProject/Markus/pull/1549 in here too, but I guess that's fine?

This deals with tweaks/features requested in issue https://github.com/MarkUsProject/Markus/issues/753, along with fixing the wonky pane heights, along with wonky image annotation creation.

This should also deal with the dragging issue from issue https://github.com/MarkUsProject/Markus/issues/1300.