Potential fix for issue #3

[talljosh]

This change fixes my issue in #3.

I haven't done any testing to see if a broader fix might be needed (e.g., do call_later() calls suffer from the same issue if they're called from within a Gtk signal handler?)

[chrysn]

I think that the quit calls don't need is_running guards -- my PR #7 can do without (even when two simultaneous events are processed, or the events don't originate in a GLib main loop execution), so probably this can as well.