NotificationServer: Add new notification windows below the lowest one

Don't stack notifications on top of each other, instead put them below
one another on the y axis.

This will obviously break if the screen fills with notifications, but
that's a FIXME for now. :^)

Servers/NotificationServer/NotificationWindow.h

@@ -38,6 +38,8 @@ public:
 
 private:
     NotificationWindow(const String& text, const String& title);
+
+    Gfx::Rect m_original_rect;
 };
 
 }