Add room bookmarking: bookmarks are always on top

Introduce a bookmark system for rooms:
a room bookmarked will always appear on top of every other
(for a given account).

Bookmarking can be done through context menu on targeted room
(as well as un-bookmarking) or through settings.json
(key roomBookmarkIDs) for permanent bookmark.
roomBookmarkIds is a {account_user_id: [room_id]} dict.

src/backend/user_files.py

@@ -291,6 +291,7 @@ class UISettings(JSONDataFile):
             "theme": "Midnight.qpl",
             "writeAliases": {},
             "zoom": 1.0,
+            "roomBookmarkIDs": {},
 
             "media": {
                 "autoLoad": True,