gpui: Add support for observing window appearance (#7294)

This PR adds support to GPUI for observing when the appearance of a
window changes.

Based on the initial work done in
https://github.com/zed-industries/zed/pull/6881.

Release Notes:

- N/A

crates/gpui/src/platform/test/window.rs

@@ -171,7 +171,7 @@ impl PlatformWindow for TestWindow {
     }
 
     fn appearance(&self) -> WindowAppearance {
-        unimplemented!()
+        WindowAppearance::Light
     }
 
     fn display(&self) -> std::rc::Rc<dyn crate::PlatformDisplay> {
@@ -276,9 +276,7 @@ impl PlatformWindow for TestWindow {
         unimplemented!()
     }
 
-    fn on_appearance_changed(&self, _callback: Box<dyn FnMut()>) {
-        unimplemented!()
-    }
+    fn on_appearance_changed(&self, _callback: Box<dyn FnMut()>) {}
 
     fn is_topmost_for_position(&self, _position: crate::Point<Pixels>) -> bool {
         unimplemented!()