libvlc: add equalizer API

libvlc: add equalizer API

Functions are provided to:

 * query the available equalizer presets
 * create a new default equalizer instance
 * create an equalizer instance from a preset
 * set individual equalizer band amplitudes
 * set equalizer pre-amplification
 * apply an equalizer to a media player

Equalizer settings can be applied whether media is playing or not,
and will automatically be retained when playing media subsequently.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>