Add a collapsible section for mod backups (*_backup folders), or move them to the end of the list

[metacubed]

Mod backups are the folders in the Staging directory with a "_backup" suffix, made using the "Create Backup" option. As of 2.5.2, all mod backups show up at the top of the mod list when you sort by priority. This pushes the actual mods down the page and reduces visibility.

Add the ability to group these backup mods into their own separator with some default name like "Mod Backups".

Alternatively, change the sorting behavior to move them to the bottom of the list when sorting by Priority.