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outdated and difficult to reason about.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/WSMenu.cpp

@@ -116,7 +116,7 @@ WSWindow& WSMenu::ensure_menu_window()
             next_item_location.move_by(0, height);
         }
 
-        auto window = make<WSWindow>(*this, WSWindowType::Menu);
+        auto window = WSWindow::construct(*this, WSWindowType::Menu);
         window->set_rect(0, 0, width, height());
         m_menu_window = move(window);
         draw();