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_macros/tests/derive_context.rs

@@ -0,0 +1,13 @@
+#[test]
+fn test_derive_context() {
+    use gpui::{App, Window};
+    use gpui_macros::{AppContext, VisualContext};
+
+    #[derive(AppContext, VisualContext)]
+    struct _MyCustomContext<'a, 'b> {
+        #[app]
+        app: &'a mut App,
+        #[window]
+        window: &'b mut Window,
+    }
+}