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outdated and difficult to reason about.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.
The only reason for the inheritance was to add FDs to the select set.
Since CNotifier is available (and now also quite useful), we can make use of it
instead, and remove the inheritance.
