Exclude shortcuts that are not available in currently active keyboard layout

This pull request goes with another one in Brackets shell repo and together they fix issue #8666.
Resolution:
Ctrl(17) is always dispatched first.
Example
http://jsfiddle.net/znakrxnk/6/

I need onKeyUp event to check ctrl is released. Then you can set property 'isCtrlDown' in your src/command/KeyboardUtils.js file.

Problem is with cleaning this property (set to false) onUp event. Can I addListener in this file?