Prepare for release 3.9.1
Post-release version bump
Tajik translation updated
Add mnemonic to Delete Image menu item

https://bugzilla.gnome.org/show_bug.cgi?id=702521
When deleting images make sure there's is actually something to delete

Fixes crashes when one tries to delete an image that was already
deleted outside of eog. The result is not nice, but it is
analogous to what "Move to Trash" does in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=702523
Updated Spanish translation
Updated Norwegian bokmål translation