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/toolbar.cpp

@@ -386,7 +386,12 @@ bool wxToolBar::Create( wxWindow *parent,
 #endif
     GtkSetStyle();
 
-    if (style & wxTB_DOCKABLE)
+    if ((style & wxTB_DOCKABLE)
+#ifdef __WXGTK3__
+        // using GtkHandleBox prevents toolbar from drawing with GTK+ >= 3.19.7
+        && gtk_check_version(3,19,7)
+#endif
+        )
     {
         m_widget = gtk_handle_box_new();