Refactoring

Some refactoring to make things look somewhat more like an actual
GObject API so that we can (hopefully) trigger gtk-doc into DTRT.

Not much luck yet, but making progress.
Fix enum_free function

Originally, joy_stick_enumerate() would return a GArray, and
joy_stick_enum_free() would free it. We changed the definition of the former
function, but forgot the latter. Do so now.
Document joystick enums

Still TODO: the joymodel enum. Later.
Mark stuff that doesn't need documentation as such.