qmlglsink: Add itemInitialized signal to QML item

qmlglsink: Add itemInitialized signal to QML item

This is useful for autoplay for example. With autoplay, it is necessary to
wait until the scene graph is fully set up. This signal is emitted once the
QML item node is ready. So, inside a connected slot, the pipeline's state
can be set to PLAYING to automatically start playback as soon as the QML
script is loaded.

https://bugzilla.gnome.org/show_bug.cgi?id=786246