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.
@MarcelGerber Yes, you're absolutely right that I need to add them in altGr.json file. Otherwise, users can't type `|` or `~` with German keyboard layout if we have `Ctrl-Alt-<` or `Ctrl-Alt-+` shortcuts.