Move all Yesod bit into a GUI module.

Since this is more of an app-with-web-interface than a web app,
(The sound comes (or will come, anyway) out of the server rather
than the client) I decided to move all the web app (Yesod) stuff
into a "GUI" submodule.

Keeping the actual library management logic and playback separate
from the web stuff will allow, among other benefits, other front-
ends to be added later.