Support for GStreamer 1.0

Configure.ac now supports an argument --with-sound, with
possible values "no", "auto" (default), "gstreamer0.10", and
"gstreamer1.0". The old --with-gstreamer option is removed.
Supplying "auto" will pick which sound library is available,
preferring 1.0 over 0.10.
Sync new GStreamer element to parent state

In GStreamer 1.0, when adding a new element to the pipeline in a signal
handler, its state has to be synced to the pipeline's state, because the
handler could be called in the middle of a state change which would
cause the state change to hang.

This was supposed to be done in GStreamer 0.10 too, but it never caused
any problems.
Refactor loading sound resource as GIO stream

There was almost identical code in two places.