Fix setting custom cursor for widgets and windows before showing them

Fix setting custom cursor for widgets and windows before showing them

If custom cursor was set before the window was created, it didn't
actually get set, and in some cases even caused a crash.
Fixed by making sure the cursor is correct when showing widget/window.

Task-number: QTBUG-27535
Change-Id: I3bc946a9c406c96af5b86869a3a54893f8980aba
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>