html5: fix ACTIVATE/DEACTIVATE getting stuck when a tab is hidden without a focus/blur pair (common on mobile Safari)

[mallyskies]

HTML5Application's ACTIVATE/DEACTIVATE tracking can get stuck DEACTIVATE forever, with nothing left to fire ACTIVATE:

1. handleWindowEvent() has had a "visibilitychange" case since Visibility-change API implementation added #1401 (closed, unmerged) / d918341, but nothing ever calls addEventListener for it (i.e. dead code since 2020). Anything hiding the tab without a blur/focus pair on the window (e.g. a tab-overview UI vs. a click into another window) never DEACTIVATEs, or DEACTIVATEs and never gets ACTIVATE back.
2. On mobile Safari, returning from the tab overview ("All Tabs") fires a single blur going out and nothing coming back: no focus, no visibilitychange. The page is frontmost and receiving touches, but hasFocus() returns false. Further, HTML5Window's preventDefault() on touchstart blocks the synthetic click some engines use internally to notice a backgrounded page regained focus, so a touch-only canvas can never leave the hidden state again after the first backgrounding.

Real-world impact
An app that suspends on DEACTIVATE (pause, mute, stop a timer) and expects ACTIVATE to resume it can become suspended for the rest of the session. We ran into this with one of our game apps that hides the game progress when not active.

Fix

1. Register the visibilitychange listener as per Page Visibility API.
2. Treat a mousedown/touchstart reaching the window's element as proof it's active, regardless of what hidden says, since these events can't happen unless the window is frontmost. Refactors the two existing "became visible" branches into one activate() so this doesn't duplicate them a third time.

Testing
project.xml:

<project>
	<app main="Main" file="ActivateTest" path="bin"/>
	<haxelib name="lime"/>
	<source path="src"/>
</project>

src/Main.hx:

import lime.app.Application;

class Main extends Application {
	public override function onWindowActivate():Void log("ACTIVATE");
	public override function onWindowDeactivate():Void log("DEACTIVATE");

	function log(s:String):Void {
		var w:Dynamic = js.Browser.window;
		if (w.__log == null) w.__log = [];
		w.__log.push(s);
	}
}

lime build html5 -debug, open html5/bin/index.html, then in the browser console, paste:

window.dispatchEvent(new Event('blur'));                // -> window.__log = ["DEACTIVATE"]
document.dispatchEvent(new Event('visibilitychange'));  // no focus event at all
window.__log;

Outputs ["DEACTIVATE"] on unpatched develop, and with the fix in place displays: ["DEACTIVATE", "ACTIVATE"].

Same process for the touch path. Reload, then:

window.dispatchEvent(new Event('blur'));
const c = document.querySelector('canvas');
const t = new Touch({identifier: 1, target: c, clientX: 10, clientY: 10});
c.dispatchEvent(new TouchEvent('touchstart', {touches: [t], targetTouches: [t], changedTouches: [t], cancelable: true, bubbles: true}));
window.__log;

Outputs ["DEACTIVATE"] on unpatched develop, and with the fix in place displays: ["DEACTIVATE", "ACTIVATE"].