youtube: add APIs to query the playback queue progress (e.g. currently playing stream 5 or 17)

[juanrubio]

/**
 * Retrieve the index in the playback queue of the stream currently selected.
 *
 * @ingroup libtizyoutube
 *
 * @param ap_gmusic The tiz_youtube handle.
 */
const char *tiz_youtube_get_current_audio_stream_index (tiz_youtube_t *ap_youtube);

/**
 * Retrieve the current length of playback queue.
 *
 * @ingroup libtizyoutube
 *
 * @param ap_gmusic The tiz_youtube handle.
 */
const char *tiz_youtube_get_current_queue_length (tiz_youtube_t *ap_youtube);

/**
 * Retrieve the current queue progress (e.g. '5 of 17', where 5 is the current
 * stream index, and 17 is the total streams in the queue).
 *
 * @ingroup libtizyoutube
 *
 * @param ap_gmusic The tiz_youtube handle.
 */
const char *tiz_youtube_get_current_queue_progress (tiz_youtube_t *ap_youtube);