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.
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