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.
Or even better, do it like this:
``` JS
var keys2 = altGrShortcuts[brackets.app.keyboard],
    keys1 = keys2 && altGrShortcuts[brackets.app.keyboard][brackets.app.keyboardType];
```