Make a Channels sub-menu, and change accelerators to be more mnemonic...

... Also removes the conflict at top menu level over M accelerator.

Make one Channel sub-menu for mono and one for stereo, and swap correct one
in, whenever the parent menu opens.

src/TrackPanel.h

@@ -830,6 +830,10 @@ protected:
    wxMenu *mRulerWaveformMenu;
    wxMenu *mRulerSpectrumMenu;
 
+   wxMenu *mChannelMenuMono;
+   wxMenu *mChannelMenuStereo;
+   wxMenu *mHiddenChannelMenu;
+
    Track *mPopupMenuTarget;
 
    friend class TrackPanelAx;