Fix bookmarks strategy

Strategy is now:

1. Get pubsub if supported
2. Get private storage and merge if we find boomarks we dont have
3. Store bookmarks to both pubsub and privatestorage

The only drawback with this strategy is, that a client that supports
only private storage cant delete bookmarks

gajim/common/modules/bookmarks.py

@@ -160,8 +160,9 @@ class Bookmarks:
                 continue
 
             if check_merge:
-                if jid not in self.bookmarks:
-                    merged = True
+                if jid in self.bookmarks:
+                    continue
+                merged = True
 
             log.debug('Found Bookmark: %s', jid)
             self.bookmarks[jid] = {
@@ -218,8 +219,7 @@ class Bookmarks:
         if storage_type is None:
             if self._pubsub_support():
                 self._pubsub_store(storage_node)
-            else:
-                self._private_store(storage_node)
+            self._private_store(storage_node)
         elif storage_type == BookmarkStorageType.PUBSUB:
             if self._pubsub_support():
                 self._pubsub_store(storage_node)