CMD+. to cancel a dialog on mac is preempted by native shell

[jasonsanjose]

When implementing a fix for #283, I found that CMD+. to cancel (see http://guides.macrumors.com/Keyboard_shortcuts#Dialog_Boxes) is handled by the native shell, but doesn't seem to be attached to any action.

It could make sense to role this fix in with the Native Menu System user story.

[njx]

Pretty sure we already have a user story around native keyboard shortcuts, but if we don't we should add one. Marking "to user story".

[jlondon]

Could find a user story in the backlog that was a great fit for those so created a new one - closing.