LibCore: Add Core::Object::add<T> helper for creating child objects

Consider the old pattern for creating a Core::Object parent and child:

    auto parent = Core::Object::construct(...);
    auto child = Core::Object::construct(..., parent);

The above was an artifact of the pre-reference-counting Object era.
Now that objects have less esoteric lifetime management, we can replace
the old pattern with something more expressive:

    auto parent = Core::Object::construct(...);
    auto child = parent->add<Core::Object>(...);

This reads a lot more naturally, and it also means we can get rid of
all the parent pointer arguments to Core::Object subclass constructors.

Libraries/LibCore/Object.h

@@ -120,6 +120,14 @@ public:
             m_parent->remove_child(*this);
     }
 
+    template<class T, class... Args>
+    inline NonnullRefPtr<T> add(Args&&... args)
+    {
+        auto t = T::construct(forward<Args>(args)...);
+        add_child(*t);
+        return t;
+    }
+
     virtual bool is_visible_for_timer_purposes() const;
 
 protected: