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outdated and difficult to reason about.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.h

@@ -4,7 +4,6 @@
 #include <AK/NonnullRefPtr.h>
 #include <AK/Optional.h>
 #include <AK/Queue.h>
-#include <AK/RefCounted.h>
 #include <LibCore/CEventLoop.h>
 #include <LibCore/CObject.h>
 #include <LibThread/Lock.h>
@@ -28,11 +27,8 @@ private:
 
 template<typename Result>
 class BackgroundAction final : public CObject
-    , public RefCounted<BackgroundAction<Result>>
     , private BackgroundActionBase {
-
     C_OBJECT(BackgroundAction);
-
 public:
     static NonnullRefPtr<BackgroundAction<Result>> create(
         Function<Result()> action,
@@ -47,7 +43,7 @@ public:
 private:
 
     BackgroundAction(Function<Result()> action, Function<void(Result)> on_complete)
-        : CObject(background_thread())
+        : CObject(&background_thread())
         , m_action(move(action))
         , m_on_complete(move(on_complete))
     {