AK: Add Eternal<T> and use it in various places.

This is useful for static locals that never need to be destroyed:

Thing& Thing::the()
{
    static Eternal<Thing> the;
    return the;
}

The object will be allocated in data segment memory and will never have
its destructor invoked.

LibGUI/GDesktop.h

@@ -6,12 +6,11 @@
 class GDesktop {
 public:
     static GDesktop& the();
+    GDesktop();
 
     String wallpaper() const;
     bool set_wallpaper(const String& path);
 
 private:
-    GDesktop();
-
     Rect m_rect;
 };