Currently the only way to get cgroup for a MetaWindow is to get it's
PID and perform a bunch of file accesses and string manipulations.
This is especially not feasible if we want to get the cgroup every
time a MetaWindow has gained or lost focus.
A solution to this is to cache the GFile for a cgroup path.
The creation and access of this GFile is handled by
`meta_window_get_unit_cgroup` function.
`meta_window_unit_cgroup_equal` is a utility function which allows
us to compare whether two MetaWindows belong to the same cgroup.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1960>
c2efe25597