QNX: Removed alternate window property

Some older QNX versions don't support SCREEN_PROPERTY_ALTERNATE_WINDOW,
so cover windows have generally been disabled on QNX.

Change-Id: Ibe4b0abc39eb8497571f88c90876571576708d79
Reviewed-by: Sérgio Martins <sergio.martins@kdab.com>
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>