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outdated and difficult to reason about.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.
