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/ios/app_state.rs

@@ -296,7 +296,7 @@ impl AppState {
                 };
                 (waiting_event_handler, event)
             }
-            (ControlFlow::Exit, _) => bug!("unexpected `ControlFlow` `Exit`"),
+            (ControlFlow::ExitWithCode(_), _) => bug!("unexpected `ControlFlow` `Exit`"),
             s => bug!("`EventHandler` unexpectedly woke up {:?}", s),
         };
 
@@ -451,7 +451,7 @@ impl AppState {
                 });
                 self.waker.start()
             }
-            (_, ControlFlow::Exit) => {
+            (_, ControlFlow::ExitWithCode(_)) => {
                 // https://developer.apple.com/library/archive/qa/qa1561/_index.html
                 // it is not possible to quit an iOS app gracefully and programatically
                 warn!("`ControlFlow::Exit` ignored on iOS");