theme: Use boxed functions instead fn pointers (#99)

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.

src/widget/list/column.rs

@@ -45,7 +45,7 @@ impl<'a, Message: 'static> ListColumn<'a, Message> {
             .spacing(12)
             .apply(iced::widget::container)
             .padding([16, 6])
-            .style(theme::Container::Custom(style))
+            .style(theme::Container::custom(style))
             .into()
     }
 }