Start new Bonjour protocol plugin

This is mostly copied from the new XMPP protocol plugin, with icons copied from
the existing Bonjour plugin.

The id has a `-nouveau` suffix so that it won't conflict with the existing one.

Testing Done:
Ran `ninja turtles`; checked that an account with the new protocol could be added, though it doesn't do anything.

Reviewed at https://reviews.imfreedom.org/r/2915/