Use idle events to update TrackPanel cursor...

... As when recording starts or stops, or the Tools button has changed, or
scrolling has changed the results of hit-test.

This reduces TrackPanel's intrusions into other code, including ProjectHistory.

Also for AdornedRulerPanel, which shares the base class CellularPanel

src/CellularPanel.h

@@ -134,6 +134,8 @@ private:
    
    void OnContextMenu(wxContextMenuEvent & event);
    
+   void OnIdle(wxIdleEvent & event);
+   
    void HandleInterruptedDrag();
    void Uncapture( bool escaping, wxMouseState *pState = nullptr );
    bool HandleEscapeKey(bool down);