Two new functions added:

 * val = libvlc_audio_get_delay( p_mi )
 * libvlc_audio_set_delay( p_mi, i_delay )

The delay is measured in microseconds.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(cherry picked from commit b8a46d564f7d6c822b8dbe945af2bcb679a0e579)