expose gst_rtsp_media_set_status

It can be useful for developers to have access to gst_rtsp_media_set_status. It gives us more control over the prepare / unprepare cycle.

In my case I wanted to keep the media prepared when a client leaves, in order to finish up some work on the pipeline. However this lead to a race condition that can be avoided by setting the media status to unprepared in klass->unprepare and setting it to prepared again on the next connection.

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