Preliminary changes for wxWidgets 3.0.1

We can't go to 3.0.1 yet as there are still build issues on
Linux and OSX.  You can get Windows to build, but there's
still some display issues.

These changes should work with wxWidgets 2.8.12 as well, so
we can take our time to get things working properly before
switching over.

src/AudacityLogger.cpp

@@ -286,7 +286,7 @@ void AudacityLogger::OnSave(wxCommandEvent & WXUNUSED(e))
    wxString fName = _("log.txt");
 
    fName = FileSelector(_("Save log to:"),
-                        NULL,
+                        wxEmptyString,
                         fName,
                         wxT("txt"),
                         wxT("*.txt"),