Gstreamer: Set custom pipeline using url

Introduced possibility to set custom pipeline using url.

Now the pipeline should be set only via QMediaPlayer::setMedia using url
with scheme "gst-pipeline".

For example, "gst-pipeline: videotestsrc ! autovideosink"

Since setting the media should be done after renderer in QML,
hence a surface is ready to be used and "qtvideosink" is available in pipelines.

To use it with QVideoWidget, setting the pipeline should be done after passing video output.

Change-Id: Id43f8681069e4bca2eafb154df2c5d446a0ca498
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Christian Strømme <christian.stromme@qt.io>