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/warning.rs

@@ -51,7 +51,7 @@ impl<'a, Message: 'static + Clone> Warning<'a, Message> {
             ])
             .align_items(Alignment::Center),
         )
-        .style(theme::Container::Custom(warning_container))
+        .style(theme::Container::custom(warning_container))
         .padding(10)
         .align_y(alignment::Vertical::Center)
         .width(Length::Fill)