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outdated and difficult to reason about.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.cpp

@@ -12,16 +12,24 @@
 #include <sys/uio.h>
 #include <unistd.h>
 
+static HashMap<int, RefPtr<ASClientConnection>> s_connections;
+
 ASClientConnection::ASClientConnection(CLocalSocket& client_socket, int client_id, ASMixer& mixer)
     : ConnectionNG(*this, client_socket, client_id)
     , m_mixer(mixer)
 {
+    s_connections.set(client_id, *this);
 }
 
 ASClientConnection::~ASClientConnection()
 {
 }
 
+void ASClientConnection::die()
+{
+    s_connections.remove(client_id());
+}
+
 void ASClientConnection::did_finish_playing_buffer(Badge<ASMixer>, int buffer_id)
 {
     (void)buffer_id;