Add onCollapse callback

[lohanbodevan]

First of all, congrats, this widget works pretty good.
Using it i had the need to execute some tasks after the bottom sheet collapse and i thought this could be useful for someone else too. So i added this optional callback: onCollapse.

Situations that can be useful
If your bottom sheet content has managed data or if you need to update some state on the parent widget, is good to know when the bottom sheet is collapsed.

[mcrovero]

Hi, that's a good point but looking at the changes I see that the onCollapse is called only after collapse() and not if the user manually collapses the bottomsheet.
What about listening for animationState changes?

[lohanbodevan]

Your are completely right @mcrovero. I guess i didn't see this animationState before. Thank you for the hint.

[sooxt98]

@lohanbodevan this pull shouldn't be close. #55

[lohanbodevan]

Hi @sooxt98 i will be glad to reopen and work on it if it still makes sense with the widget roadmap. What do you say @mcrovero ?