qmlglsink: add a QML extension plugin
Submitted by Alexandre Moreno
Link to original bug (#756082)
Description
Currently in order to make the video item (GstGLVideoItem) available to the QML engine, a sink must be created before loading the app.
This dependency could be removed by implementing a QML extension plugin (QQmlExtensionPlugin), that would install the library plugin somewhere, and the user would just need to add the path into QML2_IMPORT_PATH variable.