search provider blocks the main search thread

[brianjmurrell]

If I enable the Buddies Search Provider and then open the overview and then start typing, my typing, and indeed the whole overview gets blocked for too long -- quite a few seconds.

My understanding of how search is supposed to work in GNOME is that the search results are supposed to be incremental and as such added to the displayed results as they become available from the providers. To make this fluid and not produce the kind of lag that the Buddies search here is producing, providers are supposed to go do their work in their own thread and not block the main thread until they are ready to present the results back to the main thread.

[psukys]

Related #14