added js fullscreen support (close #604)

hxd/System.js.hx

@@ -99,7 +99,10 @@ class System {
 	}
 
 	public static function getValue( s : SystemValue ) : Bool {
-		return false;
+		return switch( s ) {
+		case IsWindowed: true;
+		default: false;
+		}
 	}
 
 	public static function exit() : Void {

hxd/Window.js.hx

@@ -152,6 +152,14 @@ class Window {
 	}
 
 	public function setFullScreen( v : Bool ) : Void {
+		var doc = js.Browser.document;
+		var elt : Dynamic = doc.documentElement;
+		if( (doc.fullscreenElement == elt) == v )
+			return;
+		if( v )
+			elt.requestFullscreen();
+		else
+			doc.exitFullscreen();
 	}
 
 	public function setCurrent() {