Move renderable cell transformation to alacritty

This refactors a large chunk of the alacritty_terminal API to expose all
data necessary for rendering uniformly through the `renderable_content`
call. This also no longer transforms the cells for rendering by a GUI
but instead just reports the content from a terminal emulation
perspective. The transformation into renderable cells is now done inside
the alacritty crate.

Since the terminal itself only ever needs to know about modified color
RGB values, the configuration for colors was moved to the alacritty UI
code.

alacritty_terminal/tests/ref.rs

@@ -84,7 +84,9 @@ struct RefConfig {
     history_size: u32,
 }
 
+#[derive(Copy, Clone)]
 struct Mock;
+
 impl EventListener for Mock {
     fn send_event(&self, _event: Event) {}
 }