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/search/field.rs

@@ -58,7 +58,7 @@ impl<'a, Message: 'static + Clone> Field<'a, Message> {
         .spacing(8)
         .align_items(iced::Alignment::Center)
         .apply(container)
-        .style(crate::theme::Container::Custom(active_style))
+        .style(crate::theme::Container::custom(active_style))
         .into()
     }
 }