[#33080] Backup icons - further integration
Contributed by Georg Kronthaler, many thanks!
* enables display of correct file icon on splash screen and in Open Recent menu
* exposes filter_backup in the python api
* enables setting BLENDERFILE_BACKUP as active filter in file browser from wm_operators.c (and from .blend in case this setting will be saved in the future)
* adds a comment to slightly misleading function name file_is_blend_backup()
* Updates icon for backup files to be more consistent with icon for .blend files
ecda79a8b8