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.

Previously, DECLINE messages let directly to the START state
for introducer and introducees.
So incoming ACCEPT and DECLINE messages needed to be ignored in START state
introducing undefined behavior into the protocol.

This is fixed with this commit by adding two additional states
to the introducer state machine as well as making use of the existing
LOCAL_DECLINED state for the introducees.

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.

Previously, the introducer would process and forward invalid messages by
the introducees. This commit adds the necessary checks and tests.

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