Need selection state for managing master checkbox

[indolering]

We need the ability to control the selection of a large number of items using the traditional checkbox UI in which a single master checkbox can be used to control selection of individual items. iron-list has most of the elements required to manually wire everything up, but it would be ideal if one could just perform databinding between iron-list and the master checkbox.

[blasten]

I see. yes that's in the radar