ListView (and other ScrollViews) should have a FocusTraversalGroup by default

[Hixie]

Proposal: We should put a FocusTraversalGroup around ListView (and the other scroll views). Currently it's a bit weird that if you put a list view in a row, with a bunch of widgets in a box next to the row, the tab order is in and out of the list.

I'm not sure if this is a good idea; there may be reasons we should not do this.

cc @gspencergoog @goderbauer

[gspencergoog]

This seems reasonable, but I worry a little bit about the effects on apps that already have customized their traversal order the way they like it, or that want a traversal order that (for instance) goes across columns of lists instead of down. If there were groups in place around the lists that they couldn't remove, it would be pretty hard to make that happen.

In the default case, though, this seems like it would be a good thing.

[Hixie]

We'd definitely want this to have an opt-out, like addAutomaticKeepAlives, addRepaintBoundaries, and addSemanticIndexes.

[tiloc]

I think there should also be some thought around how this would work with a builder-based ListView. I am currently struggling with a ListView where I am tabbing through the items on the view, but once I have tabbed to the last currently visible item on the bottom row, it doesn't build the next one, but rather starts tabbing through other elements on the screen (such as the Scaffold).