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/MouseEvent.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <LibWeb/DOM/Event.h>
+
+namespace Web {
+
+class MouseEvent final : public Event {
+public:
+    using WrapperType = Bindings::MouseEventWrapper;
+
+    static NonnullRefPtr<MouseEvent> create(String event_name, i32 offset_x, i32 offset_y)
+    {
+        return adopt(*new MouseEvent(move(event_name), offset_x, offset_y));
+    }
+
+    virtual ~MouseEvent() override {}
+
+    i32 offset_x() const { return m_offset_x; }
+    i32 offset_y() const { return m_offset_y; }
+
+protected:
+    MouseEvent(String event_name, i32 offset_x, i32 offset_y)
+        : Event(move(event_name))
+        , m_offset_x(offset_x)
+        , m_offset_y(offset_y)
+    {
+    }
+
+private:
+    virtual bool is_mouse_event() const override { return true; }
+
+    i32 m_offset_x { 0 };
+    i32 m_offset_y { 0 };
+};
+
+}