Modify and export new APNG

[ChrisZeThird]

I have to say this is such an amazing little project you have here! I'm very new to all of this and this was really helpful. On that I have a suggestion: what about being able to modify the field values like the duration, the size, etc, so you can export a new APNG? Have you ever considered trying to do that?

Additionally, would you add a way to export the frames individually?