Forum Sharing Client Backend

This MR replaces the old `ForumSharingManagerImpl` with a new one
which is based on state machines and the `ProtocolEngine`.

There is a `SharerEngine` and a `InviteeEngine` that take care of state
transitions, messages, events and trigger actions to be carried out by
the `ForumSharingManagerImpl`. This is all very similar to the
Introduction Client.

The general sharing paradigm has been changed from sharing as a state to
sharing as an action. Now the UI can allow users to invite contacts to
forums. The contacts can accept or decline the invitation. Also, the
Forum Sharing Manager is notified when users leave a forum.

Please note that you will need the UI to actually test this. It is coming up soon in another MR.

Closes #322

See merge request !170