Compositor: add BypassWindowManager hint to WindowFlags

BypassWindowManagerHint is already being used on the client-side in
QtWayland, and it would be beneficial to see this compositor-side as well.

Change-Id: I4801a0c2f14f86fcd48cc49c3f2324af4216fb6f
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>
Merge pull request #25 from xknight/master

Compositor: add BypassWindowManager hint to WindowFlags