re-add a paper-icon-button-light version

[notwaldorf]

This re-adds the paper-icon-button-light element from 1.x. It's a bit more complicated now, since we don't have type extensions, and now it's a wrapper around a native button and an iron-icon.

Most of the code is copy-pasted from https://github.com/PolymerElements/paper-icon-button/blob/1.x/paper-icon-button-light.html, changed only to:

• apply the styles to the distributed button, not the host
• set up the focus/blur listeners on the button, not the host
• set the distributed button as the ripple container.

Tests didn't exist in 1.x and don't exist now.