This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
Polybar's config format is a bit strange, and lists in particular are
annoying to handle. This enables using normal nix lists and nested
attrsets instead.
This change is not backwards-compatible, because the INI converter
converts lists of strings to space-separated values, and this does
something else. I expect that this is only relevant for the
`modules-left` etc bar setting, but that's enough to break things :(.
