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.
When you say AltGr key is down you're referring to the right Alt key, right? And your question is whether we can tell the user is pressing that right Alt key. Then the answer is no. What we get from that key down is the same as both Ctrl and Alt key down. Maybe in the shell code we can have a way to distinguish between left Alt or right Alt, but here we also need to handle JavaScript keydown  events and I don't think there is something in the event object telling you left Alt or right ALt.