Barrier-lock the image before start saving (needs testing!)

This is a bit dangerous commit. We should test saving of the document
extensively. What should be tested:

1) Saving to KRA
2) Saving to any other formats
3) Saving while some action in still executing
4) Saving when LoD mode is active
5) Saving in batch mode

The program should not crash and/or hangup. The latter is the
most probable bug.

CC:kimageshop@kde.org
Fixes T2332
BUG:361883