LibCore: Make CObject reference-counted

Okay, I've spent a whole day on this now, and it finally kinda works!
With this patch, CObject and all of its derived classes are reference
counted instead of tree-owned.

The previous, Qt-like model was nice and familiar, but ultimately also
outdated and difficult to reason about.

CObject-derived types should now be stored in RefPtr/NonnullRefPtr and
each class can be constructed using the forwarding construct() helper:

    auto widget = GWidget::construct(parent_widget);

Note that construct() simply forwards all arguments to an existing
constructor. It is inserted into each class by the C_OBJECT macro,
see CObject.h to understand how that works.

CObject::delete_later() disappears in this patch, as there is no longer
a single logical owner of a CObject.

Servers/WindowServer/WSWindowSwitcher.h

@@ -41,7 +41,7 @@ public:
     WSWindow* switcher_window() { return m_switcher_window.ptr(); }
 
 private:
-    OwnPtr<WSWindow> m_switcher_window;
+    RefPtr<WSWindow> m_switcher_window;
     Rect m_rect;
     bool m_visible { false };
     Vector<WeakPtr<WSWindow>> m_windows;