Support GetFilterIndex() when filter choice is not shown in wxOSX

Platform native behaviour is not to show a filter, but to allow all
supported types to be selectable. Make sure GetFilterIndex still is a
valid choice (and not -1 as before).

Closes https://github.com/wxWidgets/wxWidgets/pull/1976

include/wx/osx/filedlg.h

@@ -86,6 +86,7 @@ protected:
     virtual wxWindow* CreateFilterPanel(wxWindow *extracontrol);
     void DoOnFilterSelected(int index);
     virtual void OnFilterSelected(wxCommandEvent &event);
+    int GetMatchingFilterExtension(const wxString& filename);
 
     wxArrayString m_filterExtensions;
     wxArrayString m_filterNames;