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/platform.rs

@@ -1271,7 +1271,7 @@ impl ClipboardItem {
 
         for entry in self.entries.iter() {
             if let ClipboardEntry::String(ClipboardString { text, metadata: _ }) = entry {
-                answer.push_str(text);
+                answer.push_str(&text);
                 any_entries = true;
             }
         }