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outdated and difficult to reason about.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.

Applications/IRCClient/IRCAppWindow.cpp

@@ -171,7 +171,7 @@ void IRCAppWindow::setup_widgets()
     toolbar->add_action(*m_open_query_action);
     toolbar->add_action(*m_close_query_action);
 
-    auto outer_container = GWidget::construct(widget);
+    auto outer_container = GWidget::construct(widget.ptr());
     outer_container->set_layout(make<GBoxLayout>(Orientation::Vertical));
     outer_container->layout()->set_margins({ 2, 0, 2, 2 });