This allows `bar output` and `bar tray_output` to specify an output
identifier. Output names should still work as well.
This parses the output identifier from the xdg_output description,
which wlroots currently sets to `make model serial (name)`. Since this
could change in the future, all identifier comparisons are guarded by
NULL-checks in case the description cannot be parsed to an identifier.
Brian Ashworth