ENHANCEMENT: Adapt the Widgets Editor CMS UI to fit SS 3.0

Addresses https://github.com/silverstripe/silverstripe-widgets/issues/17

This is the CSS I'm using for the widgets editor in my current SS3 projects. Just wanted to pass along in case you wanted to merge this in.

Within smaller viewports a single column is used.

![widget-ui-css](https://f.cloud.github.com/assets/341209/45304/41f6e4ae-576c-11e2-9fb8-e131929b8195.png)