WIP: Modify the QmlSink integration

The original code was based in the example code from the qmlsink, and it used a QRunnable to trigger the playing / stop when the QQuickWindow hit the render thread. We can do the same by inheriting from QQuickItem without the QRunnable.

This also made easier to handle the way that the sink and the video are set setup: the original code used an objectName in the Qml and queried for that object in the QQmlEngine, now there's no need for that anymore. We use a VideoController that behaves like a normal Qml Element, and use it to set the video.

TODO: Accept complex pipelines not easily created in text mode.