Make everything interfaces

[eaburns]

Window should be an interface instead of a concrete type. Likewise with Canvas. This will make the package testable. To accomplish this, events and key and mouse constants need to be made more generic so that they aren't exposing any of their SDL heritage; a Mixer interface type should be added, and the PlayXXX functions should be added as its methods. Basically, the ui package itself should be completely independent of the implementation.

Finally, the current SDL implementations of these interfaces should be moved to an sdl/ directory that simply exports functions for making a Window and Mixer.