LibGUI+WindowServer: Create and broadcast an event when a window moves

LibWeb's Window object will need to know the OS-level position and size
of the GUI::Window for e.g. screenX, screenY, outerWidth, outerHeight.
It will also need to know about changes to that data.

Userland/Services/WindowServer/Event.h

@@ -35,6 +35,7 @@ public:
         WindowInputLeft,
         WindowCloseRequest,
         WindowResized,
+        WindowMoved,
     };
 
     Event() = default;
@@ -157,4 +158,18 @@ private:
     Gfx::IntRect m_rect;
 };
 
+class MoveEvent final : public Event {
+public:
+    MoveEvent(Gfx::IntRect const& rect)
+        : Event(Event::WindowMoved)
+        , m_rect(rect)
+    {
+    }
+
+    Gfx::IntRect const& rect() const { return m_rect; }
+
+private:
+    Gfx::IntRect m_rect;
+};
+
 }