Change my mind again about composing.

Change my mind again about composing.

Composition activities should stack above reading files, but below
things to which you might pop out to get more information (such as
checking the username of someone to @mention). So I've given them an
Activity category of their own, which has at most one occupant (like
UtilityActivity) but is separate and just below Utility.

I'm still not sure this is right, but I think it's better than before.
I'll try using it for a while and see.