Support adding beforeunload event listener

tests and comment

fix comment

Author: TomiS

Diff for: src/Webapi/Dom/Webapi__Dom__BeforeUnloadEvent.re

@@ -6,3 +6,8 @@ include Webapi__Dom__Event.Impl({ type nonrec t = t; });
 [@bs.new] external makeWithOptions : (string, Js.t({..})) => t = "BeforeUnloadEvent";
 
 [@bs.get] external returnValue : t => string = "";
+/**
+ * returnValue cannot be unset because BS cannot (afaik) bind `delete e['returnValue'];`
+ * Setting the value to undefined does not cause expected behavior.
+ */
+[@bs.set] external setReturnValue: (t, string) => unit = "returnValue";

Diff for: src/Webapi/Dom/Webapi__Dom__Window.re

@@ -94,6 +94,8 @@ module Impl = (T: {type t;}) => {
 
   [@bs.send.pipe : t_window] external addPopStateEventListener : ([@bs.as "popstate"] _, Dom.popStateEvent => unit) => unit = "addEventListener";
   [@bs.send.pipe : t_window] external removePopStateEventListener : ([@bs.as "popstate"] _, Dom.popStateEvent => unit) => unit= "removeEventListener";
+  [@bs.send.pipe : t_window] external addBeforeUnloadEventListener : ([@bs.as "beforeunload"] _, Dom.beforeUnloadEvent => unit) => unit = "addEventListener";
+  [@bs.send.pipe : t_window] external removeBeforeUnloadEventListener : ([@bs.as "beforeunload"] _, Dom.beforeUnloadEvent => unit) => unit= "removeEventListener";
 
   [@bs.set] external setOnLoad : (t_window, unit => unit) => unit = "onload"; /* use addEventListener instead? */
 };

Diff for: tests/Webapi/Dom/Webapi__Dom__BeforeUnloadEvent__test.re

@@ -21,3 +21,4 @@ stopPropagation(event);
 
 /* BeforeUnloadEvent */
 let _ = returnValue(event);
+setReturnValue(event, "Test");

Diff for: tests/Webapi/Dom/Webapi__Dom__Window__test.re

@@ -70,5 +70,7 @@ let _ = Window.resizeTo(120, 300, window);
 let _ = Window.scroll(0.0, 0.0, window);
 let _ = Window.scrollBy(10.0, (-10.0), window);
 let _ = Window.scrollTo(120.5, 300.3, window);
+Window.addBeforeUnloadEventListener((_: BeforeUnloadEvent.t) => (), window);
+Window.removeBeforeUnloadEventListener((_: BeforeUnloadEvent.t) => (), window);
 Window.stop(window);
 Window.setOnLoad(window, () => print_endline("load"));