Manage more than one device from the UI

[singular0]

Storage is already scoped per device public key and the channel cache follows it, so most of the
work is done. What is missing is the UI.

Tasks:

• List known devices with their names and when each was last seen.
• Switch between them without going through the connect dialog.
• Delete a device's history and cached channels when it is no longer wanted.
• Keep the identity rule intact: a target address is not a device identity, and only SELF_INFO
  supplies the public key that storage is scoped by.