Fix Scaffold.of(context) calls and update Scaffold constructors

[percula]

The new version of Scaffold in flutter does not have a nullOk parameter, so I removed that.

I also ran into an issue where ContextualActionBar was calling Scaffold.of(context) but since it was a sibling in the Stack, it could not find the correct Scaffold. I updated this so it is within the Scaffold, and the Scaffold builds either the appBar or contextualAppBar depending on if ActionMode is enabled or not.

[percula]

I also added the option for action to be taken on the selected items when the close button is pressed. My application uses LongPressDraggable so I have to implement selection and deselection myself. This allows me to take advantage of a highlighted observable value I already am using, rather than rebuilding every selected and deselected widget when the ActionMode is enabled/disabled.

[De-Morgan]

There was a reason the contextual appbar was stacked. The reason being that you can have multiple contextual appbar. Run the "Whatsapp" sample code to see what i mean here.

[percula]

I see. So to avoid the issue I described, "where ContextualActionBar was calling Scaffold.of(context) (contextual_action_bar.dart line 216) but since it was a sibling in the Stack, it could not find the correct Scaffold", it looks like you wrap everything in another Scaffold? I wonder if there is a way to avoid wrapping it with another Scaffold.

[De-Morgan]

What's the usecase for this ContextualCloseAction?. The material design guideline doesn't have this specification. Personally i will prefer a callback to handle this or better still use
ContextualAction instead of create an entirely new type of widget.

[percula]

So initially I tried to just pass HandleItems<T>, but the type was getting erased (sorry, not an expert in Dart but that appears to be what was happening). Wrapping it in ContextualAction<T> got rid of that issue, but if you know of a way to prevent the type from being erased, definitely would like to go that route. The use case for this is simply to do an action on the list of selected items when the close button is selected, just like a ContextualAction does.

[percula]

Closing this as I've added a few things that are custom to my needs.