Clean up QScreen::grabWindow()

Clean up QScreen::grabWindow()

Handle 0 WId parameter as meaning "desktop window"/whole screen.
Also, re-add the default values for the grab area, both for
convenience and compatibility with QPixmap::grabWindow() in Qt4.
Update the screenshot example so it doesn't comlain about usage of
deprecated QPixmap::grabWindow().

Change-Id: I2ad229113ddb8ded0388f2ebc0e8c703c6657f1f
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>