Add exit code to ControlFlow::Exit (#2100)

* Add exit code to control flow and impl on linux

* Fix examples to have an exit code

* Fix doc examples to use an exit code

* Improve documentation wording on the exit code

* Add exit code example

* Add exit code on windows

* Change i32 as exit code to u8

This avoids nasty surprises with negative numbers on some unix-alikes
due to two's complement.

* Fix android usages of ControlFlow::Exit

* Fix ios usages of ControlFlow::Exit

* Fix web usages of ControlFlow::Exit

* Add macos exit code

* Add changelog note

* Document exit code on display server disconnection

* Revert "Change i32 as exit code to u8"

This reverts commit f88fba0253b45de6a2ac0c3cbcf01f50503c9396.

* Change Exit to ExitWithCode and make an Exit const

* Revert "Add exit code example"

This reverts commit fbd3d03de9c2d7516c7a63da489c99f498b710df.

* Revert "Fix doc examples to use an exit code"

This reverts commit daabcdf9ef9e16acad715c094ae442529e39fcbc.

* Revert "Fix examples to have an exit code"

This reverts commit 0df486896b8d106acf65ba83c45cc88d60d228e1.

* Fix unix-alike to use ExitWithCode instead of Exit

* Fix windows to use ExitWithCode rather than Exit

* Silence warning about non-uppercase Exit const

* Refactor exit code handling

* Fix macos Exit usage and recover original semantic

* Fix ios to use ExitWithCode instead of Exit

* Update documentation to reflect ExitWithCode

* Fix web to use ExitWithCode when needed, not Exit

* Fix android to use ExitWithCode, not Exit

* Apply documenation nits

* Apply even more documentation nits

* Move change in CHANGELOG.md under "Unreleased"

* Try to use OS error code as exit code on wayland

src/platform_impl/macos/event_loop.rs

@@ -155,11 +155,11 @@ impl<T> EventLoop<T> {
     where
         F: 'static + FnMut(Event<'_, T>, &RootWindowTarget<T>, &mut ControlFlow),
     {
-        self.run_return(callback);
-        process::exit(0);
+        let exit_code = self.run_return(callback);
+        process::exit(exit_code);
     }
 
-    pub fn run_return<F>(&mut self, callback: F)
+    pub fn run_return<F>(&mut self, callback: F) -> i32
     where
         F: FnMut(Event<'_, T>, &RootWindowTarget<T>, &mut ControlFlow),
     {
@@ -176,7 +176,7 @@ impl<T> EventLoop<T> {
 
         self._callback = Some(Rc::clone(&callback));
 
-        autoreleasepool(|| unsafe {
+        let exit_code = autoreleasepool(|| unsafe {
             let app = NSApp();
             assert_ne!(app, nil);
 
@@ -192,9 +192,11 @@ impl<T> EventLoop<T> {
                 drop(self._callback.take());
                 resume_unwind(panic);
             }
-            AppState::exit();
+            AppState::exit()
         });
         drop(self._callback.take());
+
+        exit_code
     }
 
     pub fn create_proxy(&self) -> Proxy<T> {