These don't affect client visibility.

It is possible that a remote DECLINE message arrives short before the
user responds to the introduction.
This will cause a ProtocolStateException which (for now) is just caught
and a generic (existing) error message will be shown.

A fake ACTIVATE message would cause us to activate the transport keys
before the contact has received our auth message,
which would compromise forward secrecy.

When the user attempts an introduction, instead of the introduction
message input field, an explanatory text will be shown and the
introduction can not be made until the last one has been finished.

Closes #308