ESC key will destroy the windows mentioned at issue 211.

This change implements generic routine gtkutil_destroy_on_esc ()
and adds invocations of it in the requisite window-handling places.

src/fe-gtk/plugingui.c

@@ -219,6 +219,7 @@ plugingui_open (void)
 	plugin_window = mg_create_generic_tab ("Addons", _(DISPLAY_NAME": Plugins and Scripts"),
 														 FALSE, TRUE, plugingui_close, NULL,
 														 500, 250, &vbox, 0);
+	gtkutil_destroy_on_esc (plugin_window);
 
 	view = plugingui_treeview_new (vbox);
 	g_object_set_data (G_OBJECT (plugin_window), "view", view);