Backend progress, to be tested

 - Moved default sorting YAML files to package `sorting_defaults`
 - Rearranged `SortingPreferencesBackend` to have file loading functions at the top
 - Worked on `SortingPreferencesBackend` functions (to be tested)
 - Changed default return type for `SectionalizeMapList()` to `new LinkedList` for compatibility
 - Added `Inheritee` field and accessors for `PreferencesClass`