NoteTrack: Implement Silence and InsertSilence

Additionally, fix the UI portions of these, and fix Trim for note tracks
(the code already existed, but due to flags would not work).  As PRL
requested, this is gated only behind USE_MIDI.

src/commands/CommandFlag.h

@@ -55,6 +55,7 @@ enum CommandFlag : unsigned long long
    NotPausedFlag         = 0x400000000ULL, // jkc
    HasWaveDataFlag       = 0x800000000ULL, // jkc
    PlayableTracksExistFlag = 0x1000000000ULL,
+   AudioTracksSelectedFlag = 0x2000000000ULL,
 
    NoFlagsSpecifed        = ~0ULL
 };