- the ability to create new GSources in Perl could be handy, and wouldn't be
  hard to implement (a hash in place of GSourceFuncs).
- GEnum type for G_PRIORITY_VALUES?
- can't implement g_idle_remove_by_data because ... well, how would you
  search for the data value?
- proper cleanup for bad parameter types in Glib::Object::new?
  - would require either iterating over the list twice or cleaning up before
    the croak.  the latter is probably the way to go.
  - there is no way to clean up if gperl_sv_from_value() fails.
- gperl_value_from_sv should change to void return since it croaks on error.
  (can't do this without breaking ABI)
