Add ibus_config_watch/unwatch.

Add ibus_config_watch/unwatch.

Currently IBusConfig proxy is notified every config value change.
e.g. ibus-m17n will get notified even when ibus-anthy's config values
are changed.

With this patch, IBusConfig proxy will be able to restrict
notification by calling ibus_config_watch().  For example, after:

ibus_config_watch (config, "engine/M17N/hi/inscript", NULL);

it will be notified value changes only under
"engine/M17N/hi/inscript" section.

BUG=none
TEST=manually with modified ibus-m17n

Review URL: https://codereview.appspot.com/5091045