Don't reset the window type on window show

We were resetting the window type on show, this was breaking some
deployments, since it used to be possible to set the xcb properties from
the user application.
This change requires the user to at least start using the
QtPlatformHeaders before overriding the window properties with the values
set there.

Change-Id: Ic58919a42b6d532035c7ec6158a086b85666a22f
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>