Hide rooms that have a defined room type

I have a room with a custom type that's only for holding data, and
doesn't need to be shown in the room list. Currently the spec is a bit
vague about what clients should do, but hiding them is probably fine
for now.

src/models/roomtreemodel.h

@@ -50,6 +50,7 @@ public:
         IconRole,
         AttentionRole, /**< Whether there are any notifications. */
         FavouriteRole, /**< Whether the room is favourited. */
+        RoomTypeRole, /**< The room's type. */
     };
     Q_ENUM(EventRoles)
     explicit RoomTreeModel(QObject *parent = nullptr);