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.
