Fixes bug #42.

Of course, not all devices support Bluetooth...

This follows the principle of least surprise: the default identity
doesn't change when a new identity is created.

This allows the oldest LocalAuthor to be used as the default.

Hopefully this gives users a better understanding of whether their
messages and contacts are accessible than "Briar is running" and "Quit".

This give a simple and usable interface; we can worry about collapsing
blocks of read messages later.