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/nav_bar.rs

@@ -36,7 +36,7 @@ where
         .apply(container)
         .height(Length::Fill)
         .padding(11)
-        .style(theme::Container::Custom(nav_bar_style))
+        .style(theme::Container::custom(nav_bar_style))
 }
 
 #[must_use]