For windows that do not have WS_EX_TOOLWINDOW set, the WINDOWPLACEMENT
API uses workspace/available aera coordinates. Introduce a helper
function to return the offset and use that.
Task-number: QTBUG-43872
Change-Id: I329c640f180524699b45b855b4583f447c4a0987
Reviewed-by: Oliver Wolff <oliver.wolff@theqtcompany.com>