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outdated and difficult to reason about.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/WSWindowManager.cpp

@@ -57,7 +57,7 @@ WSWindowManager::WSWindowManager()
     };
 
     u8 system_menu_name[] = { 0xc3, 0xb8, 0 };
-    m_system_menu = make<WSMenu>(nullptr, -1, String((const char*)system_menu_name));
+    m_system_menu = WSMenu::construct(nullptr, -1, String((const char*)system_menu_name));
 
     int appIndex = 1;
     for (const auto& app : apps) {