This is more general, and necessary if the custom theming is dynamically
generated.
Iced's builtin theme also uses `Box`ed `Fn`, or `Rc` where clone is
required, so this seems reasonable.
Only `Text` is left using `fn`, since it needs to be `Copy`. Hopefully
that can be changed in Iced at some point.
`::custom` methods are added to make these variants a little more
convenient to construct. This replaces a couple `From` implementations,
which are potentially problematic with a generic.
I think it's best to only include things in the Iced fork that can't be
done without forking Iced, and/or are expected to be merged upstream.
The theme doesn't seem to be either, so it fits more appropriately in
libcosmic.
That should make it easier to keep up with upstream, and it may help to
have all the Cosmic parts in one place.
Based on https://github.com/pop-os/iced commit ad9026e.
