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outdated and difficult to reason about.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.

Servers/AudioServer/ASClientConnection.h

@@ -15,6 +15,8 @@ public:
     ~ASClientConnection() override;
     void did_finish_playing_buffer(Badge<ASMixer>, int buffer_id);
 
+    virtual void die() override;
+
 private:
     virtual OwnPtr<AudioServer::GreetResponse> handle(const AudioServer::Greet&) override;
     virtual OwnPtr<AudioServer::GetMainMixVolumeResponse> handle(const AudioServer::GetMainMixVolume&) override;