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/lib.rs

@@ -5,8 +5,13 @@ pub use iced_style;
 pub use iced_winit;
 
 pub mod font;
+pub mod theme;
 pub mod widget;
 
+pub use theme::Theme;
+pub type Renderer = iced::Renderer<Theme>;
+pub type Element<'a, Message> = iced::Element<'a, Message, Renderer>;
+
 #[derive(Clone, Copy, Debug)]
 pub enum WindowMsg {
     Close,