Issue #138: Backporting dropbuttons from D8

This PR addresses https://github.com/backdrop/backdrop-issues/issues/138 by backporting both https://drupal.org/node/1608878 and https://drupal.org/node/1480854.

It converts all operations links in all Backdrop, with the exception of image styles, to avoid conflicting with the CMI branch at #backdrop-issues/2.