Make wxSetInputFocusToXWindow a member instead of free function

Also add a couple of workarounds for problems when using Xvfb.

See https://github.com/wxWidgets/wxWidgets/pull/1845

tests/validators/valnum.cpp

@@ -211,6 +211,16 @@ void NumValidatorTestCase::Interactive()
         return;
 #endif // __WXMSW__
 
+#ifdef __WXGTK20__
+    // Travis CI fails without this!
+    if ( IsAutomaticTest() )
+    {
+        wxFrame* frame = wxDynamicCast(wxTheApp->GetTopWindow(), wxFrame);
+        frame->SetFocus();
+        frame->Raise();
+    }
+#endif // __WXGTK20__
+
     // Set a locale using comma as thousands separator character.
     wxLocale loc(wxLANGUAGE_ENGLISH_UK, wxLOCALE_DONT_LOAD_DEFAULT);