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outdated and difficult to reason about.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.

Libraries/LibThread/BackgroundAction.cpp

@@ -28,7 +28,7 @@ static int background_thread_func()
 static void init()
 {
     s_all_actions = new LibThread::Lockable<Queue<Function<void()>>>();
-    s_background_thread = new LibThread::Thread(background_thread_func);
+    s_background_thread = &LibThread::Thread::construct(background_thread_func).leak_ref();
     s_background_thread->set_name("Background thread");
     s_background_thread->start();
 }