This commit 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 invitiation. Also, the
Forum Sharing Manger is notified when users leave a forum.
Closes#322
* If the user has already declined, we don't show that the other
introducee has declined as well. The backend doesn't have that information, so
this is compatible with the principle of showing what we know.
* If the user has already accepted or hasn't yet responded, we show the
decline response in the private conversation with the introducer. If
the user hasn't yet responded, we hide the accept/decline buttons
in the introduction request message.
Messages an introducee receives in a `FINISHED` state are now being
ignored and deleted.
Closes#295
Remove the Toast that always says 'Message Sent' and show graphical
indicators instead that show either:
* message is waiting to be sent
* message was sent (or requested)
* message was delivered
