Window navigation

Implements a window switcher that works like alt/cmd-tab.

Invoke with: `F5` or ``CTRL-` `` or ``CMD-` `` (backtick)

Cycle through windows with arrow up/down or when invoked with CMD/CTRL + backtick then hitting backtick again will cycle forward, hitting SHIFT+backtick will cycle backward.

When invoked with CTRL/CMD releasing CTRL/CMD will accept selection. Escape will always cancel. Enter accepts selection.

![Screen Shot 2013-02-27 at 7 34 18 PM](https://f.cloud.github.com/assets/467450/203975/e253394c-8157-11e2-81cf-791ec4a35b03.png)