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outdated and difficult to reason about.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/Piano/main.cpp

@@ -14,8 +14,8 @@
 int main(int argc, char** argv)
 {
     GApplication app(argc, argv);
-    AClientConnection audio_connection;
-    audio_connection.handshake();
+    auto audio_client = AClientConnection::construct();
+    audio_client->handshake();
 
     auto window = GWindow::construct();
     window->set_title("Piano");