If we don't do this, then boolean/int/list keys will seem to sort of
work (but defaulting to false/0/[] instead of the correct schema
defaults), but string keys will return null, which will usually cause
exceptions or crashes.
https://bugzilla.gnome.org/show_bug.cgi?id=611214
Although methods like gconf_client_get/set_bool() and such are usable
from gjs, get_list/set_list is not, since there's only one method for
all list types. So ShellGConf wraps GConfClient and adds separate
typed list methods.
Also, add a detailed "changed" signal that can easily be connected to
from js, since we can't currently use gconf_client_notify_add()
directly.
