LibWeb+WebContent: Add abstraction layer for event loop and timers

Instead of using Core::EventLoop and Core::Timer directly, LibWeb now
goes through a Web::Platform abstraction layer instead.

This will allow us to plug in Qt's event loop (and QTimer) over in
Ladybird, to avoid having to deal with multiple event loops.

Userland/Libraries/LibWeb/Platform/EventLoopPlugin.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Function.h>
+#include <LibWeb/Platform/EventLoopPlugin.h>
+
+namespace Web::Platform {
+
+EventLoopPlugin* s_the;
+
+EventLoopPlugin& EventLoopPlugin::the()
+{
+    VERIFY(s_the);
+    return *s_the;
+}
+
+void EventLoopPlugin::install(EventLoopPlugin& plugin)
+{
+    VERIFY(!s_the);
+    s_the = &plugin;
+}
+
+EventLoopPlugin::~EventLoopPlugin() = default;
+
+}