Code review comment for lp:~nick-dedekind/mir/trusted_sessions

Maybe quite a few changes.

1) The mir_trust_session_add_trusted_session should now be called after a mir_trust_session_start. Allows trusted helpers to dynamically add sessions to the trust session. This is more for phase 2.

use case:
Gallery -> ContentHub -> Facebook/Flickr/Twitter
   a) user starts gallery
   a.1) user view: gallery app pops up
   b) user selects picture and clicks "share"
   c) content hub creates destination picker
   c.1) user view: destination picker surface pops up
   d) user selects Facebook
   d.1) user view: Facebook pops up.

While it's possible for the trust session to know about the content hub + gallery, it doesn't know the user will select facebook when it starts.

2) I've separated out the trust session notifications from the SessionListener into it's own TrustSessionListener.

3) Added a trust session container, which has bidirectional mapping between process ids and TrustSessions. This allows quick lookup when we need to add/remove sessions from a trust session.
3.1) mir::Session now has no links to trust session/trusted helper.