Compensate for wxW 3 tab navigation deficiencies on Mac...

... using char hook event handlers.  We don't need to go the extreme length
of patching wxWidgets source.

src/widgets/wxPanelWrapper.cpp

@@ -0,0 +1,37 @@
+//
+//  wxPanelWrapper.cpp
+//  Audacity
+//
+//  Created by Paul Licameli on 6/25/16.
+//
+//
+
+#include "../Audacity.h"
+#include "wxPanelWrapper.h"
+
+IMPLEMENT_CLASS(wxPanelWrapper, wxPanel)
+
+wxPanelWrapper::wxPanelWrapper(wxWindow * parent, wxWindowID id,
+                           const wxPoint & pos,
+                           const wxSize & size,
+                           long style)
+: wxPanel(parent, id, pos, size, style)
+{}
+
+void wxPanelWrapper::OnCharHook(wxKeyEvent &event)
+{
+   if (event.GetKeyCode() == WXK_TAB) {
+      wxWindow::FindFocus()->Navigate(
+         event.ShiftDown()
+         ? wxNavigationKeyEvent::IsBackward
+         :  wxNavigationKeyEvent::IsForward
+      );
+      return;
+   }
+
+   event.Skip();
+}
+
+BEGIN_EVENT_TABLE(wxPanelWrapper, wxPanel)
+   EVT_CHAR_HOOK(wxPanelWrapper::OnCharHook)
+END_EVENT_TABLE()