[Build] Don't require a specific nunit version for Frontend-Tests
IChatView: add participant list property and AddMessage method

Add a property that can return the list of the names of the participants
in the chat and a method that adds a line to the chat history to IChatView.

This is the plumbing for the refactored nickname completion.
add the centralized tab completion classes

Add the new, centralized tab completion classes. This removes the need for
code duplication between the frontends.
use the new nick completion classes

Hook up the GNOME and System.Windows.Forms frontends to the new nickname
completion classes.
code style improvements
exchange NickCompleters in ApplyConfig(), not in _NickCompletion()
remove PersonLookup*() from GroupChatView

These methods were only used by the old-style refactoring.
add unit tests for the new tab completion
make IChatView.Participants a list of PersonModels, not strings