Let Configuration decide if all paths have been selected to be backed up or not

lib/core/configuration.dart

@@ -331,6 +331,10 @@ class Configuration {
 
   Future<void> setPathsToBackUp(Set<String> newPaths) async {
     await _preferences.setStringList(foldersToBackUpKey, newPaths.toList());
+    final allFolders = (await FilesDB.instance.getLatestLocalFiles())
+        .map((file) => file.deviceFolder)
+        .toList();
+    await setSelectAllFoldersForBackup(newPaths.length == allFolders.length);
     SyncService.instance.onFoldersSet(newPaths);
     SyncService.instance.sync();
   }

lib/ui/backup_folder_selection_page.dart

@@ -136,9 +136,6 @@ class _BackupFolderSelectionPageState extends State<BackupFolderSelectionPage> {
                     : () async {
                         await Configuration.instance
                             .setPathsToBackUp(_selectedFolders);
-                        await Configuration.instance
-                            .setSelectAllFoldersForBackup(
-                                _selectedFolders.length == _allFolders.length);
                         Bus.instance.fire(BackupFoldersUpdatedEvent());
                         Navigator.of(context).pop();
                       },