Introducing QQuickWindow::scheduleRenderJob()

[ChangeLog][QtQuick][QQuickWindow] Added
QQuickWindow::scheduleRenderJob(), a convenience alternative to the
equivalent signals for one-shot tasks.

Change-Id: I5e4f0d67d5223f7fd77bca394e2a85810fadd335
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>