Support setting initial position for TLW with GTK3

(cherry picked from commit 5fb89cb86e)

src/gtk/toplevel.cpp

@@ -682,10 +682,17 @@ bool wxTopLevelWindowGTK::Create( wxWindow *parent,
 
     PostCreation();
 
-#ifndef __WXGTK3__
-    if (pos != wxDefaultPosition)
+    if (pos.IsFullySpecified())
+    {
+#ifdef __WXGTK3__
+        GtkWindowPosition windowPos;
+        g_object_get(m_widget, "window-position", &windowPos, NULL);
+        if (windowPos == GTK_WIN_POS_NONE)
+            gtk_window_move(GTK_WINDOW(m_widget), m_x, m_y);
+#else
         gtk_widget_set_uposition( m_widget, m_x, m_y );
 #endif
+    }
 
     // for some reported size corrections
     g_signal_connect (m_widget, "map_event",