NotificationManager rework

Rework notifications manager to no longer be a singleton, but a component of controller.

The dependency on it for neochat room and connection is also removed.

src/qml/RoomPage.qml

@@ -186,7 +186,7 @@ Kirigami.Page {
         target: RoomManager
         function onCurrentRoomChanged() {
             if (root.currentRoom && root.currentRoom.isInvite) {
-                root.currentRoom.clearInvitationNotification();
+                Controller.clearInvitationNotification(root.currentRoom.id);
             }
         }