history operator (Ctrl + Alt + Z).
This will only show paint operations now while in an image paint mode.
The caveat is that user can delete previous paint operations too (even
on images not on the canvas currently) so it needs some care. This is
consistent with regular undo behaviour though.
Sculpting also suffers from lack of Undo history support, this will be
added in a separate commit.
93684d5b5e