Replace Closed event with CloseRequested and Destroyed (#476)

* Replace Closed event with CloseRequested and Destroyed

Implements #434

The existing Closed event had ambiguous meaning, both in name and in
cross-platform behavior. Closed is now split into two more precise events:

* CloseRequested - the window has been requested to close, most commonly by
having clicked the window's close button. Whether or not you respond by
closing the window is up to you.

* Destroyed - the window has been destroyed, and can no longer be safely
used.

Most notably, now you can reliably implement classic patterns like
prompting the user to save their work before closing, and have the
opportunity to perform any necessary cleanup.

Migrating to the new API is straightforward. In most cases, you can simply
replace all existing usages of Closed with CloseRequested. For more
information, see the example programs, particularly handling_close and
multiwindow.

iOS applications must replace all usages of Closed with Destroyed, and
require no other changes.

src/platform/ios/mod.rs

@@ -51,12 +51,12 @@
 //!  - applicationWillResignActive is Focused(false)
 //!  - applicationDidEnterBackground is Suspended(true)
 //!  - applicationWillEnterForeground is Suspended(false)
-//!  - applicationWillTerminate is Closed
+//!  - applicationWillTerminate is Destroyed
 //!
-//! Keep in mind that after Closed event is received every attempt to draw with
+//! Keep in mind that after Destroyed event is received every attempt to draw with
 //! opengl will result in segfault.
 //!
-//! Also note that app will not receive Closed event if suspended, it will be SIGKILL'ed
+//! Also note that app will not receive Destroyed event if suspended, it will be SIGKILL'ed
 
 #![cfg(target_os = "ios")]
 
@@ -459,7 +459,7 @@ fn create_delegate_class() {
             // immidiatly after jump
             state.events_queue.push_front(Event::WindowEvent {
                 window_id: RootEventId(WindowId),
-                event: WindowEvent::Closed,
+                event: WindowEvent::Destroyed,
             });
             longjmp(mem::transmute(&mut jmpbuf),1);
         }