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/theme/expander.rs

@@ -52,7 +52,7 @@ impl std::default::Default for Appearance {
 
 /// A set of rules that dictate the [`Appearance`] of a container.
 pub trait StyleSheet {
-    type Style: Default + Copy;
+    type Style: Default;
 
     /// Produces the [`Appearance`] of a container.
     fn appearance(&self, style: Self::Style) -> Appearance;