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)

src/widget/list/column.rs

@@ -45,7 +45,7 @@ impl<'a, Message: 'static> ListColumn<'a, Message> {
             .spacing(12)
             .apply(iced::widget::container)
             .padding([16, 6])
-            .style(theme::Container::Custom(style))
+            .style(theme::Container::custom(style))
             .into()
     }
 }

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]

src/widget/search/field.rs

@@ -58,7 +58,7 @@ impl<'a, Message: 'static + Clone> Field<'a, Message> {
         .spacing(8)
         .align_items(iced::Alignment::Center)
         .apply(container)
-        .style(crate::theme::Container::Custom(active_style))
+        .style(crate::theme::Container::custom(active_style))
         .into()
     }
 }

src/widget/spin_button/mod.rs

@@ -83,7 +83,7 @@ impl<'a, Message: 'static> SpinButton<'a, Message> {
         .align_y(Vertical::Center)
         .width(Length::Units(95))
         .height(Length::Units(32))
-        .style(theme::Container::Custom(container_style))
+        .style(theme::Container::custom(container_style))
         .apply(Element::from)
         .map(on_change)
     }

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)