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/toolbars/MixerToolBar.h

@@ -30,7 +30,6 @@ class MixerToolBar:public ToolBar {
 
    void Create(wxWindow * parent);
 
-   void RecreateTipWindows();
    void UpdatePrefs();
    void UpdateControls();
    void SetMixer(wxCommandEvent &event);