feature_request: add method for fluid_synth_all_notes_off

[insomnimus]

Fluidlite exposes a function for turning all notes off:

// In fluid_synth.c
/*
 * fluid_synth_all_notes_off
 *
 * put all notes on this channel into released state.
 */
int
fluid_synth_all_notes_off(fluid_synth_t* synth, int chan)
{ /* ... */ }

It doesn't seem that there's any method on fluidlite::Synth for this, can we add it?

I sort of need it for my MIDI player (plmidi).