Cleanup RoomList UI ops

* Mark unused parameters, and remove unnecessary widgets passed to callbacks.
* Remove unused `purple_roomlist_[gs]et_protocol_data`.
* Remove unused list of `PurpleRoomList`s.
* Inline `update_roomlist` using GObject macros.
* Remove use of `PurpleRoomListUiOps->destroy`.
This can be achieved through object destruction notifiers.
I'm not sure how the room list would ever get destroyed in Finch though, as the
widget seems to hold a ref to it, which is cleared when the list is destroyed.
But the list can't be destroyed until said ref is dropped.
* Replace `PurpleRoomlistUiOps->in_progress` with notify signals.

Testing Done:
Compile only. Might have done some tests, but I wrote this a while back and don't remember.

Reviewed at https://reviews.imfreedom.org/r/785/