Fix window double borrows (#23739)

Fix bugs caused by the window context PR, where the window could be on
the stack and is then requested from the App.
This PR also adds derive macros for `AppContext` and `VisualContext` so
that it's easy to define further contexts in API code, such as
`editor::BlockContext`.

Release Notes:

- N/A

crates/gpui/src/gpui.rs

@@ -129,7 +129,7 @@ pub use elements::*;
 pub use executor::*;
 pub use geometry::*;
 pub use global::*;
-pub use gpui_macros::{register_action, test, IntoElement, Render};
+pub use gpui_macros::{register_action, test, AppContext, IntoElement, Render, VisualContext};
 pub use http_client;
 pub use input::*;
 pub use interactive::*;