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/windows/event_loop.rs

@@ -341,7 +341,7 @@ impl<T: 'static> EventLoop<T> {
         };
 
         // We wait until we've checked for an exit status before clearing the
-        // application callback, in case we need to dispatch a LoopDestroyed event
+        // application callback, in case we need to dispatch a LoopExiting event
         //
         // # Safety
         // This pairs up with our call to `runner.set_event_handler` and ensures