Re-implement note tracks in MixerBoard

This commit adds note tracks into the mixerboard.  It's done as a separate
slider this time instead of via subclasses (as PRL requested), so which
should be easier to use.

This also changes some of the gaurds to EXPERIMENTAL_MIDI_OUT from
USE_MIDI, as it's meaningless to have the note track code in mixerboard
when it cannot do anything (depends on methods that exist behind
EXPERIMENTAL_MIDI_OUT).

src/commands/CommandFlag.h

@@ -54,6 +54,7 @@ enum CommandFlag : unsigned long long
    PausedFlag            = 0x200000000ULL, // jkc
    NotPausedFlag         = 0x400000000ULL, // jkc
    HasWaveDataFlag       = 0x800000000ULL, // jkc
+   PlayableTracksExistFlag = 0x1000000000ULL,
 
    NoFlagsSpecifed        = ~0ULL
 };