Show only the channel-related menu items appropriate to mono or stereo, and...

... Changed accelerators to avoid conflict:
for mono tracks, R&ight
for stereo tracks, Split Stereo to &Mono

src/TrackPanel.h

@@ -820,6 +820,9 @@ protected:
 #endif
 
    wxMenu *mWaveTrackMenu;
+   size_t mChannelItemsInsertionPoint;
+   bool mShowMono;
+
    wxMenu *mNoteTrackMenu;
    wxMenu *mTimeTrackMenu;
    wxMenu *mLabelTrackMenu;