[#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
PATCH: [#32989] Activate backup files filter in File Browser
Contributed by Georg Kronthaler, many thanks!
(I just moved the icon to a different place reserved for file browser icons)
* enables the filtering of backup files in the file browser
* adds a 'filter backup files'-icon to the filter buttons
* adds new icons for backup files in list and thumbnail view
* enables file preview for the backup files
