Updated room sorting

Change the LastActivity sort order to Activity and update to a more flexible way of sorting based on an order from model roles.
Add options to actually switch between Alphabetical and Activity

Based on some old work by @tdfischer 

implements #103

src/models/roomtreemodel.h

@@ -47,6 +47,8 @@ public:
         IsDirectChat, /**< Whether this room is a direct chat. */
         DelegateTypeRole,
         IconRole,
+        AttentionRole, /**< Whether there are any notifications. */
+        FavouriteRole, /**< Whether the room is favourited. */
     };
     Q_ENUM(EventRoles)
     explicit RoomTreeModel(QObject *parent = nullptr);