LibWeb: Add a DOM Event class (instead of events being simple strings)

This patch adds the Event base class, along with a MouseEvent subclass.
We now dispatch MouseEvent objects for mousedown, mouseup and mousemove
and these objects have the .offsetX and .offsetY properties.

Both of those properties are hard-coded at the moment. This will be
fixed in the next patch. :^)

Libraries/LibWeb/DOM/Event.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <AK/String.h>
+#include <LibWeb/Bindings/Wrappable.h>
+
+namespace Web {
+
+class Event
+    : public RefCounted<Event>
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::EventWrapper;
+
+    static NonnullRefPtr<Event> create(String event_name)
+    {
+        return adopt(*new Event(move(event_name)));
+    }
+
+    virtual ~Event() {}
+
+    const String& name() const { return m_event_name; }
+
+    virtual bool is_mouse_event() const { return false; }
+
+protected:
+    Event(String event_name)
+        : m_event_name(move(event_name))
+    {
+    }
+
+private:
+    String m_event_name;
+};
+
+}