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outdated and difficult to reason about.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/WindowServer/WSMenuManager.cpp

@@ -27,7 +27,7 @@ WSMenuManager::~WSMenuManager()
 
 void WSMenuManager::setup()
 {
-    m_window = make<WSWindow>(*this, WSWindowType::Menubar);
+    m_window = WSWindow::construct(*this, WSWindowType::Menubar);
     m_window->set_rect(WSWindowManager::the().menubar_rect());
 }