derived widget
- Add History::set_notify_enabled() for usage by the frontend
(for example, when loading from file, you'll want to call
set_notify_enabled(false) before starting)
git-svn-id: svn://svn.gna.org/svn/sgpemv2/trunk@823 3ecf2c5c-341e-0410-92b4-d18e462d057c