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/HTML/EventLoop/EventLoop.h

@@ -84,7 +84,7 @@ private:
 
     JS::VM* m_vm { nullptr };
 
-    RefPtr<Core::Timer> m_system_event_loop_timer;
+    RefPtr<Platform::Timer> m_system_event_loop_timer;
 
     // https://html.spec.whatwg.org/#performing-a-microtask-checkpoint
     bool m_performing_a_microtask_checkpoint { false };