LibCore: Add Core::Object::try_add<T>(...)

This is a fallible version of add<T>(...) that returns ErrorOr<T>.
It can be used together with TRY() to handle allocation failures when
instantiating new Core::Objects.

Userland/Libraries/LibCore/Object.cpp

@@ -73,14 +73,20 @@ void Object::event(Core::Event& event)
     }
 }
 
-void Object::add_child(Object& object)
+ErrorOr<void> Object::try_add_child(Object& object)
 {
     // FIXME: Should we support reparenting objects?
     VERIFY(!object.parent() || object.parent() == this);
+    TRY(m_children.try_append(object));
     object.m_parent = this;
-    m_children.append(object);
     Core::ChildEvent child_event(Core::Event::ChildAdded, object);
     event(child_event);
+    return {};
+}
+
+void Object::add_child(Object& object)
+{
+    MUST(try_add_child(object));
 }
 
 void Object::insert_child_before(Object& new_child, Object& before_child)

Userland/Libraries/LibCore/Object.h

@@ -129,6 +129,8 @@ public:
     void stop_timer();
     bool has_timer() const { return m_timer_id; }
 
+    ErrorOr<void> try_add_child(Object&);
+
     void add_child(Object&);
     void insert_child_before(Object& new_child, Object& before_child);
     void remove_child(Object&);
@@ -164,6 +166,14 @@ public:
         return child;
     }
 
+    template<class T, class... Args>
+    inline ErrorOr<NonnullRefPtr<T>> try_add(Args&&... args)
+    {
+        auto child = TRY(T::try_create(forward<Args>(args)...));
+        TRY(try_add_child(*child));
+        return child;
+    }
+
     virtual bool is_visible_for_timer_purposes() const;
 
     bool is_being_inspected() const { return m_inspector_count; }