Add a GST_MESSAGE_REQUEST_STATE that can be posted by element when they would
like to have the application change the state of the pipeline. the primary use
case is to pause the pipeline when an audio mixer is mixing a higher priority
stream but it can also be used for other purposes.

Add some docs and a unit test.

Implement the REQUEST_STATE message in gst-launch.

API: gst_message_new_request_state()
API: gst_message_parse_request_state()
API: GST_MESSAGE_REQUEST_STATE