gpui: Activate the window example along with the Quit action (#30790)

Make the gpui examples more consistent by activating the window upon
startup.
Most of the examples have 

```rust
activate(true) 
```

so this one should as well.

Make it easier to exit the example with the `cmd-q` KeyBinding

Release Notes:

- N/A

crates/gpui/examples/input.rs

@@ -26,6 +26,7 @@ actions!(
         Paste,
         Cut,
         Copy,
+        Quit,
     ]
 );
 
@@ -741,5 +742,7 @@ fn main() {
                 cx.activate(true);
             })
             .unwrap();
+        cx.on_action(|_: &Quit, cx| cx.quit());
+        cx.bind_keys([KeyBinding::new("cmd-q", Quit, None)]);
     });
 }