Add new API to libvlc for persistent equalizer settings.

Add new API to libvlc for persistent equalizer settings.

New methods are provided to:

 - get the available equalizer presets;
 - get the available frequency bands (useful for creating a UI);
 - create a new default equalizer, or create a new equalizer from a preset;
 - release a previously created equalizer;
 - get/set preamp and individual frequency amplification values;
 - apply equalizer settings to a media player.

Equalizer settings are persistent, can be made whether media is currently playing or not, and will automatically be applied to subsequently played media.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>