Tweet: Filter mentions more strictly

We just ignore multiple mentions of the same user as well as when the
authenticated user get mentioned.
That unfortunately makes it necessary to pass the used account instance
to `load_from_json`.

See issue #97.
TweetUtils: Use array in work_array

No need to use a GList here.
manpage: Add --tweet parameter

It exists, even though it does not come up in --help for some reason.
gdk pixbuf vapi: Remove _from_stream_async functions

We redefine them in our code anyway.
FavoritesTimeline: Remove unfavorited tweets on_leave

See #103
UserStream: Don't identifty status_withheld events as delete events