Add function for setting the startup ID for the next window that will be shown.

This is useful for applications being requested to show a second window
(e.g. via DBus)

Change-Id: I04add2aa82dce63e854ba0f1c020274ed586bf1f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Add missing Q_DECL_OVERRIDE

Change-Id: Iffa196043d345d0b839a45635c0500d1abc3199c
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>