QmlDesigner: Fix all states only showing base state in States Editor

Let the State Editor also use the new activateState method from
QmlModelView (stateChanged by default does now nothing). This allowed
us to get rid of the setCurrentStateBack method. Also fix deletion of
states - the "decision" which state should be the new one is now done
in StatesEditorView.

Reviewed-by: Thomas Hartmann
Reviewed-by: Christiaan Janssen