Add base interface for objects with display properties.
This will allow the formatter to check if strings coming from
(for example) users in channels should be formatted specially,
such as by colouring or linking them.
Issues #424 and DMDirc/Plugins#507
This is for the interfaces/trivial implementation classes that
DMDirc plugins can interact with. Eventually plugins will build
against the API instead of the DMDirc client itself, which will
allow for remote/detachable clients (amongst other things).