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/icon.rs

@@ -207,7 +207,7 @@ impl<'a> Icon<'a> {
 
     fn svg_element<Message: 'static>(&self, handle: svg::Handle) -> Element<'static, Message> {
         svg::Svg::<Renderer>::new(handle)
-            .style(self.style)
+            .style(self.style.clone())
             .width(self.width.unwrap_or(Length::Units(self.size)))
             .height(self.height.unwrap_or(Length::Units(self.size)))
             .content_fit(self.content_fit)