Extend tree recovery to group folders

When ‑loadTree:rootNode: returns NO, recover group folders as well as
RSS and OpenReader folders by reattaching them under the root folder.

A user reported in issue #1784 losing group folders which just went
invisible but were still present in database.

Vienna/Sources/Main window/FoldersTree.m

@@ -159,12 +159,9 @@ -(void)reloadDatabase:(NSArray *)stateArray
         // recover from problems by putting missing folders under root node
         NSArray *allFolders = [[[Database sharedManager] arrayOfAllFolders]
                                sortedArrayUsingSelector:@selector(folderNameCompare:)]; // all RSS and group folders
-        NSArray *installedFolders = [self folders:0];  // RSS folders already present
         for (Folder *folder in allFolders) {
-            if ((folder.type == VNAFolderTypeRSS || folder.type == VNAFolderTypeOpenReader)
-                && ![installedFolders containsObject:folder])
-            {
-                (void)[[TreeNode alloc] init:self.rootNode atIndex:-1 folder:folder canHaveChildren:NO];
+            if (![[self children:0] containsObject:folder]) { // not in already installed RSS and group folders
+                (void)[[TreeNode alloc] init:self.rootNode atIndex:-1 folder:folder canHaveChildren:folder.isGroupFolder];
             }
         }
         // save the state after recovery