Add option to allow creation of backups to be disabled
When using a version control system the backup (example.sch~) files
created by libgeda become annoying. They can be ignored by most
VCSs however they still sit there and don't serve any purpose; any
mistakes can be recovered from a previous version.
This patch adds a configuration option "no-backup-files" which
suppresses the creation of backup files. It is disabled by default
so there is no change to the previous behaviour unless this option
is activated.
Closes-bug: lp-762894