Reworked the TipPanel used with ASliders

It was producing "ghost" windows on OSX in wx3.  These were
supposed to be hidden, but they weren't any longer and after
reviewing TipPanel, I realized that there was a separate
code path for OSX entirely...must've gone back to some of the
earliest versions.

Now all platforms use the same bit of code.

src/AudacityApp.cpp

@@ -296,8 +296,6 @@ void QuitAudacity(bool bForce)
       }
    }
 
-   LWSlider::DeleteSharedTipPanel();
-
    ModuleManager::Get().Dispatch(AppQuiting);
 
 #ifdef EXPERIMENTAL_SCOREALIGN