selection changes - not easy to distinguish user-caused vs code-caused

[robrez]

Description

Currently, we learn of selection-change events through data-bindings only. Selections can change via internal (to iron list) event handlers or through the selection-related functions. We'd currently have to resort to some unpleasant techniques for separating internal (user-driven) changes from external (code-driven) changes.

https://github.com/webcomponents/gold-standard/wiki/Property-Change-Events