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/spin_button/mod.rs

@@ -83,7 +83,7 @@ impl<'a, Message: 'static> SpinButton<'a, Message> {
         .align_y(Vertical::Center)
         .width(Length::Units(95))
         .height(Length::Units(32))
-        .style(theme::Container::Custom(container_style))
+        .style(theme::Container::custom(container_style))
         .apply(Element::from)
         .map(on_change)
     }