Add completion closure for dismissal

[siberian1967]

In many cases the calling controller would like to be able to be notified or run code when the FloatingActonSheetController is dismissed. Currently dismissActionSheet method has this closure but it is private and its also called internally making it a less simple change to reliably make and PR.

Feature request is to allow the calling code to set this closure so that when the FloatingActionSheetController is dismissed its closure is executed, allowing the calling code to react.

The button based callbacks are almost an analog but if you tap outside the buttons to close without action there is no closure to catch this action.

[giuseppebruno]

I'm also interested in this feature... Right now, waiting for an update, the only solution is to comment the line 383 inside FloatingActionSheetController.swift. This disables the dismissal function by clicking outside of the ActionSheetController. Of course it is not a great solution...