Avoid blank menubar/toolbar when using wx[MT]B_DOCKABLE with GTK+ 3.20

A GTK+ bug involving GtkHandleBox somehow prevents drawing of its child
since 3.19.7.  Avoid this by disabling the docking functionality, which
is preferable to having the bar be blank.
See #17539

src/gtk/menu.cpp

@@ -120,7 +120,12 @@ void wxMenuBar::Init(size_t n, wxMenu *menus[], const wxString titles[], long st
 
     m_menubar = gtk_menu_bar_new();
 
-    if (style & wxMB_DOCKABLE)
+    if ((style & wxMB_DOCKABLE)
+#ifdef __WXGTK3__
+        // using GtkHandleBox prevents menubar from drawing with GTK+ >= 3.19.7
+        && gtk_check_version(3,19,7)
+#endif
+        )
     {
         m_widget = gtk_handle_box_new();
         gtk_container_add(GTK_CONTAINER(m_widget), m_menubar);