media: Farstream 0.1.1 compatibility fix
Because of change in Farstream, purple_media_get_codecs() can return
NULL until any needed codec configuration parameters have been
discovered.
As a consequence we have to wait until both local candidates and codec
parameters are gathered before firing sipe_media_call->candidates_prepared_cb().
Also some functions and fields were renamed to better depict their
purpose:
* sipe_media_call->candidates_prepared_cb to
sipe_media_call->stream_initialized_cb
* candidates_prepared_cb() to stream_initialized_cb()
* sipe_backend_candidates_prepared() to sipe_backend_stream_initialized()
and gets new sipe_backend_stream * argument
This fixes broken video calls.