Change of user colour

[phil]

The user colour should be changeable from within the session.

• The host has to change the colour of the user (e.g. through a call to a method of obby::user)
• Send it to the client (e.g. obby_user_colour:34:23:12)
• obby::buffer must be able to interpret this
• Same within the client, but a packet needs to be sent to the host
• The new colours need to be checked if they conflict [code in server_buffer::on_auth, perhaps to buffer::check_colour()], use in client and host before setting it

[phil]

Fixed in [224]