Update event on LiveList

[gcostaapps]

Hi,

First I want to thank you for maintaning this great library.

I'm developing a live query funtionality and I faced the ParseLiveListWidget, although it provides live query functionalities it mixes presentation code with logic.

There is a way to update the objects listening to the LiveList stream but right now when a object is updated it doesn't send a event, only for creating and deleting objects.

I solved this issue adding the ParseLiveListUpdateEvent. I've already submited a pr to resolve this. PR #606

If you can review it and merge I think it would benefit people like me who doesn't want to mix logic with presentation.

(I'm using the null-safety branch)

[fischerscode]

I am not sure if this solves your problem, but back when coding the ParseLiveList part, I intended the ParseLiveListElement to handle in object updates.

As it should be fully compatible adding you additional Event might be a good idea in order to give developers more options.

I will have a look at your PR soon.

[fischerscode]

I've merged your PR, thanks for your contribution.