This searches everything in PurplePerson and all of it's contacts, but does not yet cover tags. I am planning on covering that in another review request.
Right now the matching is just using strstr, but will will update this as part of PIDGIN-17737.
I also tried to figure out how to get focus back to the list view, but didn't come up with any viable solutions.

Ran the unit tests and did a bunch of searches in the contact list with the demo protocol plugin.