Merge branch 'safe-show-message'

Show message box from wxSafeShowMessage() in the other ports too, if
possible.

Currently this is done using wxMessageBox() if it can be sure that it's
safe to call and only MSW uses native function which is always safe to
call. Ideal would be to also use a native function under Mac, where it
should also be the case, but this doesn't seem to work for whichever
reason.

See https://github.com/wxWidgets/wxWidgets/pull/2270

src/common/config.cpp

@@ -98,7 +98,7 @@ wxConfigBase *wxConfigBase::Set(wxConfigBase *pConfig)
 wxConfigBase *wxConfigBase::Create()
 {
   if ( ms_bAutoCreate && ms_pConfig == NULL ) {
-    wxAppTraits * const traits = wxTheApp ? wxTheApp->GetTraits() : NULL;
+    wxAppTraits * const traits = wxApp::GetTraitsIfExists();
     wxCHECK_MSG( traits, NULL, wxT("create wxApp before calling this") );
 
     ms_pConfig = traits->CreateConfig();