Save incremental backup patch

I have implemented what I think deevad is explaining in the previous
post. I do a simple QFile::copy(fileName, backupFilename) before the
working file is saved. Please see the attached patch. PS! I'm a bit
uncertain about the fix I have done to the code that handles if you
are working on a backup file. Please test that as well!

BUG:314214
CCMAIL:christer.stenbrenden@gmail.com

Thanks to Christer Stenbrenden for the patch!