Add keyboard-accessible means of reordering widgets

Widgets on the admin page have this, and we need it in the customizer as well.

Currently in WordPress, the feature is provided via the "Enable Accessibility" link:

> ![01-enable-accessibility-mode](https://f.cloud.github.com/assets/134745/1289782/fa6cefc2-3026-11e3-88a8-25b43d9009fa.png)

The inline widget form is disabled and an "Edit" link appears at `/wp-admin/widgets.php?widgets-access=on`:

> ![02-accessibility-mode-on-edit-link](https://f.cloud.github.com/assets/134745/1289783/fd1a8748-3026-11e3-90f9-0b85b7aafe97.png)

Then a standalone page (e.g. at `/wp-admin/widgets.php?widgets-access=on&editwidget=categories-2&sidebar=sidebar-1&key=0`) appears which contains the widget form, followed by a sidebar selector and a position within each sidebar:

> ![03-accessible-editing](https://f.cloud.github.com/assets/134745/1289787/0459dbd0-3027-11e3-8705-fcfd301a34e4.png)

There has got to be a better way to do this. The menu editor has a better model, I think:

> ![menus__wordpress_develop__wordpress](https://f.cloud.github.com/assets/134745/1289848/fab2d43c-3027-11e3-8f79-e73c79d43e1f.png)

We could add these links to the `widget-control-actions`.

Related to #26