Rename LoopDestroyed to LoopExiting

Considering the possibility of re-running an event loop via run_ondemand
then it's more correct to say that the loop is about to exit without
assuming it's going to be destroyed.

src/platform_impl/linux/x11/mod.rs

@@ -492,7 +492,7 @@ impl<T: 'static> EventLoop<T> {
 
             let mut dummy = self.control_flow;
             sticky_exit_callback(
-                Event::LoopDestroyed,
+                Event::LoopExiting,
                 self.window_target(),
                 &mut dummy,
                 &mut callback,