DOM manipulation should use Polymer APIs

[lomori]

I was having weird results with sortable elements that were added dynamically. After spending quite some time debugging my code I've noticed that the underlying Sortable.js module does quite a few DOM inserts and removals directly, without using Polymer.dom calls. Shouldn't it if this library is being used inside a custom web component? See https://www.polymer-project.org/1.0/docs/devguide/local-dom.html.