gst-discoverer: use state changes instead of ASYNC_DONE.

gst-discoverer: use state changes instead of ASYNC_DONE.

And monitor no_more_pads.

With live sources such as rtsp, uridecodebin only creates its
child decodebins between PAUSED and PLAYING.

This means that the ASYNC_DONE it posts when getting NO_PREROLL
in its change_state method gets immediately propagated by the
GstBin parent class, as opposed to a situation where a
decodebin has been added to it already, and has posted ASYNC_START.

The proposed solution, instead of simply waiting for ASYNC_DONE,
and finishing prematurely in that case, waits for three conditions
to be true:

* the uridecodebin needs to have emitted no_more_pads
* its current state must be PAUSED if not live, PLAYING otherwise
* There must be no "pending subtitle pads", ie pads where we haven't
  received tags yet.

All these conditions are checked in the message handler, as we
post custom messages on it when we get subtitle tags or no_more_pads.

https://bugzilla.gnome.org/show_bug.cgi?id=783257