LibWeb+WebContent+Browser: Plumb visibility state from GUI to web pages

OOPWV now reacts to show/hide events and informs LibWeb about the state
change. This makes visibilitychange events fire when switching tabs. :^)

Userland/Libraries/LibWeb/HTML/VisibilityState.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Web::HTML {
+
+enum VisibilityState {
+    Hidden,
+    Visible,
+};
+
+}