Offer options when trying to overwrite externally-modified file

When the user tries to do a "Save" operation on an open file that was
since modified externally, currently Okular prohibits it. We can make
this a bit more user-friendly by instead explaining the situation
clearly and offering the user some options:
- Yes, really overwrite the changes made in the other program
- Instead save the unsaved changes as a new file elsewhere
- Abort

CCBUG: 402017