Trigger dataChanged in timeline model on stateChanged in model manager

Allow the timeline model to clear its caches if the model manager changes
states. The timeline model caches for example event categories which
become invalid if the manager assumes the state EMPTY.

Task-number: QTCREATORBUG-10706

Change-Id: I10529ac56d39b169b850a8b5250fd36cb65a57ba
Reviewed-by: Kai Koehne <kai.koehne@digia.com>