Support the showIsFullScreen style hint in widgets.

Support the showIsFullScreen style hint in widgets.

Now QWidget::show() will be the same as QWidget::showFullScreen() if the
style hint is set.
This is consistent with QQuickView now.

De-inline related methods to make it easier to change them later
without breaking compatibility.

Change-Id: I843ac6f846428217bfc5dc9f1d0a554de9d0c08f
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>