RAII wrapper for invocations similar to CallbacksGroup

[TheRealKaup]

Users can accidentally forget to call CancelInvocation() on their yet to be called invoked function in their class destructors.

For instance, this is how it's currently done (example from the Button widget):

	~Button()
	{
		if (m_downInvocation != nullptr)
		{
			engine.time.CancelInvocation(m_downInvocation);
		}
	}