Fix typo.
Mark database dirty when images are deleted.
DuplicateMerger: don't delete destination image.

When a duplicate is not a different file but just a duplicate entry in
the database, only remove the database entry, not the image file.

CCBUG: 320473
DuplicateMerger: add option to just block image from DB.

CCBUG: 320635
Fix typo, add doc.
Handle identical description in ImageInfo::merge.

I.e. just leave description as-is if both images have the same
description.
Correctly handle stacks in ImageInfo::merge().

BUG: 320441