Gstreamer: Allow to use gst specific features

Gstreamer implementation is located in a plugin.

In case if there is a need to use some internal features
QGstreamerPlayerControl and QGstreamerPlayerSession have been moved
from plugin to QtMultimediaGstTools library.

It allows to request media player control from QMediaService
and use it as QGstreamerPlayerControl.

Change-Id: I8e45480c4cd718b90b90d14a42bc56f6f924eae5
Reviewed-by: Christian Stromme <christian.stromme@qt.io>