The output_id is more of an opaque identifier for the monitor, based on
its underlying ID from the windowing system. Since we also use the term
"output_id" for the output's index, rename our use of the opaque cookie
"output_id" to "winsys_id".
Right now this just has all of the files in one directory. We'll
be introducing more structure to this in the future, and build
a proper backend system.