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.

examples/cosmic/src/window.rs

@@ -277,7 +277,7 @@ impl Window {
                 .spacing(16),
             )
             .padding([20, 24])
-            .style(theme::Container::Custom(list::column::style)),
+            .style(theme::Container::custom(list::column::style)),
         )
         .padding(0)
         .style(theme::Button::Transparent)