Rethink utility status of some activities.

Rethink utility status of some activities.

ComposeToplevel and ComposeReply are very similar in this environment,
so I think it's wrong that one should be a utility activity and the
other not. The current status mimics real Monochrome, with
ComposeToplevel and ComposeReply corresponding to adding to a file and
sending a message to a user respectively; but in real Mono, those two
activities are much more different from each other than they are here,
so I don't think the same rule should apply.

Conversely, I'm promoting ThreadFile to a _non_-utility activity, on
the basis that all activities involving viewing a file of actual toots
ought to have the same status, or else it's confusing. In particular,
it's annoying to try to examine a user seen in someone's thread, and
then find you can't easily come back to the place you were at in the
thread itself.