Inbox

This implements an inbox list, invoked with `sx-inbox`, and notifications with `sx-inbox-notification`
It builds on #173 , which I'm aware you've been working on. So when you push to that one I'll merge back here.

Hitting RET on comment notifications doesn't do anything yet. But it works on answers.
Hitting `v` works on anything.
 
Don't we need to bump the required version number?