Make show() default to sane sizing behaviour based on the platform.

Make show() default to sane sizing behaviour based on the platform.

Traditionally it's been hard to write a Qt app that behaves sanely
across embedded and desktop platforms, i.e. defaults to fullscreen on
embedded and non-fullscreen on desktop. For Qt 5 we can fix this by
making the behaviour of the default QWindow::show() be customizable by
the platform plugin.

If the application developer wants to override this behaviour he can
still use the explicit showFullScreen(), showNormal() etc functions.

Change-Id: I26a907b404058e345d841c818daefbb57a26d3fd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>