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 f88fba0253.

* Change Exit to ExitWithCode and make an Exit const

* Revert "Add exit code example"

This reverts commit fbd3d03de9.

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

This reverts commit daabcdf9ef.

* Revert "Fix examples to have an exit code"

This reverts commit 0df486896b.

* 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/web/event_loop/runner.rs

@@ -84,7 +84,7 @@ impl<T: 'static> Runner<T> {
     }
 
     fn handle_single_event(&mut self, event: Event<'_, T>, control: &mut root::ControlFlow) {
-        let is_closed = *control == root::ControlFlow::Exit;
+        let is_closed = matches!(*control, root::ControlFlow::ExitWithCode(_));
 
         (self.event_handler)(event, control);
 
@@ -409,7 +409,7 @@ impl<T: 'static> Shared<T> {
             RunnerEnum::Destroyed => return,
         }
 
-        let is_closed = *control == root::ControlFlow::Exit;
+        let is_closed = matches!(*control, root::ControlFlow::ExitWithCode(_));
 
         // Don't take events out of the queue if the loop is closed or the runner doesn't exist
         // If the runner doesn't exist and this method recurses, it will recurse infinitely
@@ -456,7 +456,7 @@ impl<T: 'static> Shared<T> {
                     ),
                 }
             }
-            root::ControlFlow::Exit => State::Exit,
+            root::ControlFlow::ExitWithCode(_) => State::Exit,
         };
 
         match *self.0.runner.borrow_mut() {