Include Cosmic theme in libcosmic, instead of in Iced fork

I think it's best to only include things in the Iced fork that can't be
done without forking Iced, and/or are expected to be merged upstream.
The theme doesn't seem to be either, so it fits more appropriately in
libcosmic.

That should make it easier to keep up with upstream, and it may help to
have all the Cosmic parts in one place.

Based on https://github.com/pop-os/iced commit ad9026e.

src/theme/expander.rs

@@ -0,0 +1,56 @@
+use iced_core::{Background, Color};
+
+/// The appearance of a [`Expander`](crate::native::expander::Expander).
+#[derive(Clone, Copy, Debug)]
+pub struct Appearance {
+    /// The background of the [`Expander`](crate::native::expander::Expander).
+    pub background: Background,
+
+    /// The border radius of the [`Expander`](crate::native::expander::Expander).
+    pub border_radius: f32,
+
+    /// The border width of the [`Expander`](crate::native::expander::Expander).
+    pub border_width: f32,
+
+    /// The border color of the [`Expander`](crate::native::expander::Expander).
+    pub border_color: Color,
+
+    /// The background of the head of the [`Expander`](crate::native::expander::Expander).
+    pub head_background: Background,
+
+    /// The text color of the head of the [`Expander`](crate::native::expander::Expander).
+    pub head_text_color: Color,
+
+    /// The background of the body of the [`Expander`](crate::native::expander::Expander).
+    pub body_background: Background,
+
+    /// The text color of the body of the [`Expander`](crate::native::expander::Expander).
+    pub body_text_color: Color,
+
+    /// The color of the close icon of the [`Expander`](crate::native::expander::Expander).
+    pub toggle_color: Color,
+}
+
+impl std::default::Default for Appearance {
+    fn default() -> Self {
+        Self {
+            background: Color::WHITE.into(),
+            border_radius: 10.0, //32.0,
+            border_width: 1.0,
+            border_color: [0.87, 0.87, 0.87].into(), //Color::BLACK.into(),
+            head_background: Background::Color([0.87, 0.87, 0.87].into()),
+            head_text_color: Color::BLACK,
+            body_background: Color::TRANSPARENT.into(),
+            body_text_color: Color::BLACK,
+            toggle_color: Color::BLACK,
+        }
+    }
+}
+
+/// A set of rules that dictate the [`Appearance`] of a container.
+pub trait StyleSheet {
+    type Style: Default + Copy;
+
+    /// Produces the [`Appearance`] of a container.
+    fn appearance(&self, style: Self::Style) -> Appearance;
+}