Introduce ViewContext::on_window_should_close

This is a new callback that can be used to interrupt closing the window
when the user has unsaved changes.

crates/gpui/src/platform.rs

@@ -93,6 +93,7 @@ pub trait Window: WindowContext {
     fn on_event(&mut self, callback: Box<dyn FnMut(Event) -> bool>);
     fn on_active_status_change(&mut self, callback: Box<dyn FnMut(bool)>);
     fn on_resize(&mut self, callback: Box<dyn FnMut()>);
+    fn on_should_close(&mut self, callback: Box<dyn FnMut() -> bool>);
     fn on_close(&mut self, callback: Box<dyn FnOnce()>);
     fn prompt(&self, level: PromptLevel, msg: &str, answers: &[&str]) -> oneshot::Receiver<usize>;
     fn activate(&self);