* 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
This Introduction BSP Client uses its own group to communicate with
existing contacts. It uses four types of messages to facilitate
introductions: the introduction, the response, the ack and the abort.
The protocol logic is encapsulated in two protocol engines, one for the
introducer and one for the introducee. The introduction client keeps the
local state for each engine, hands messages over to the engines and
processes the result and state changes they return.
