add a settings_schema and make icons configurable

[liljenstolpe]

Just like my suggestion to use XDG in devdeck, I think it would be good to use it in this project to allow the configuration of the light icons. Also, to use XDG to locate the assets directory for those icons.

I propose that we use XDG_CONFIG_PATH to locate a 'devdeck/assets' directory for the icons. We also probably want to use a settings_schema like is found in a few other devdeck projects.

[liljenstolpe]

I'm having some problems using the schema definition logic in this plug-in. How/where is the schema defined for cerebus for the key light plugin?