Select items

[Westbrook]

Fixes #436
Relates to #361 (would make any batch selecting therein also not be slow)

Relies on:
Polymer/polymer#4699
Polymer/polymer#4705

Adds code paths and tests to support selecting all of the items in the list or batch selecting a subset of the list. Further the ideas introduces in #437, this relies on extensions to both Polymer 1.x and 2.x to empower array-selector with selectItems and selectAll functionality and forks the application of that code as needed in this hybrid component.

[Westbrook]

Based on the response from the Polymer team in Polymer/polymer#4699 (comment) would a PR that included a custom iron-list-selector element be something that would be acceptable? Trying to see whether I should push back on that comment or whether that seems reasonable that this level of responsibility be passed down to the iron-list component.

I guess one alternate possibility would be to have a stand alone component/repo that handled these responsibilities and then require it in iron-list but I’m not sure the right place to expect that to be managed. Thoughts?

[Westbrook]

Would that still block this PR on 1.x not having this capability? Can we move to a non-hybrid iron-list, yet? Just trying to figure out the right way to move this forward. Thanks for any suggestions.

[keanulee]

I would lean towards just baking iron-list-selector functionality right into iron-list if that's possible. Having different code paths to maintain hybrid compatibility would be acceptable.