Remove backup files

Dia normally saves a copy of an old saved file if it is overwritten via the "File Save" function. This clutters the directory. Since the common behaviour of programs is to not save backups in a separate location (also for security reasons), this patch removes this feature and therefore removes backup files when dia is sure to have the newly written file in the correct location.

From Debian package, see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=173650

Closes: #100 (closed)