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.cpp

@@ -103,6 +103,7 @@ BEGIN_EVENT_TABLE(CellularPanel, OverlayPanel)
     EVT_SET_FOCUS(CellularPanel::OnSetFocus)
     EVT_KILL_FOCUS(CellularPanel::OnKillFocus)
     EVT_CONTEXT_MENU(CellularPanel::OnContextMenu)
+    EVT_IDLE(CellularPanel::OnIdle)
 END_EVENT_TABLE()
 
 CellularPanel::CellularPanel(
@@ -472,6 +473,12 @@ void CellularPanel::OnContextMenu(wxContextMenuEvent & WXUNUSED(event))
    DoContextMenu();
 }
 
+void CellularPanel::OnIdle(wxIdleEvent &event)
+{
+   event.Skip();
+   HandleCursorForPresentMouseState();
+}
+
 /// Handle mouse wheel rotation (for zoom in/out, vertical and horizontal scrolling)
 void CellularPanel::HandleWheelRotation( TrackPanelMouseEvent &tpmEvent )
 {