Redo TP_UpdateStatusMessage...

... Rename it as SetStatus; make it part of AudacityProject not
TrackPanelListener; and use a roundabout event signalling to cause the timer
restart.

Because SetStatus will be one of very few things left in AudacityProject, but
the timer handling will be part of another class decoupled from it.

And TrackPanelListener won't really be needed: TrackPanel will not need to
pretend it doesn't know what an AudacityProject is.

src/menus/TrackMenus.cpp

@@ -1384,7 +1384,7 @@ void OnTrackClose(const CommandContext &context)
 
    if (isAudioActive)
    {
-      project.TP_DisplayStatusMessage(
+      project.SetStatus(
          _("Can't delete track with active audio"));
       wxBell();
       return;