[ChangeLog][QtGui][QGuiApplication] Add support for -icon command line
argument on X11, add -qwindowicon on all platforms.
Change-Id: Iacc602466699bf634d8b34aab7ed73c83fd9844f
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>