purple: add support for extra groupchat settings (helps with SIPE)
This adds channel settings prefixed by purple_. For example jabber now
has purple_room and purple_server which are decomposed variants of our
own 'room' setting. Okay, that doesn't sound very useful.
It also adds some sync from the values returned by chat_info_defaults()
- so if the plugin figures something out in there, we save it in our
own settings.
In the case of SIPE this adds a new setting, purple_uri, which can be
set with the ma-chan:// uri for a persistent chat.
This solves the issue with the SIPE plugin only knowing how to do name
lookups after doing 'chat list' - now it just needs to work once, and we
save the real URI in our settings.
