Bug1075, and define and use new event type for TrackPanel timer ticks...

... Thus allowing TrackPanel.cpp to work without including Lyrics headers.
Also eliminates some of its direct notification of MixerBoard.

src/LyricsWindow.h

@@ -32,6 +32,7 @@ class LyricsWindow : public wxFrame {
 
    void OnStyle_BouncingBall(wxCommandEvent &evt);
    void OnStyle_Highlight(wxCommandEvent &evt);
+   void OnTimer(wxCommandEvent &event);
 
    AudacityProject *mProject;
    Lyrics *mLyricsPanel;