Upgrade Window Level for stay on top widget on OS X

Currently setting the WindowStaysOnTopHint makes the window appear over
its children e.g. a QMessageBox.

This patch aims to provide a more sensible level for stay on top widget

[ChangeLog][QtWidget][OS X] Fixes an issue where stay on top widgets
would cover their own children

Task-number: QTBUG-36178
Change-Id: Ie86b0e0244d2fdee3638d4ab576445ef190d4bc6
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>