Shortcut validation doesn’t consider different keyboard layouts

[zoul]

Test case: Using the US keyboard layout, assign the ^2 shortcut to a menu item and switch to the Czech keyboard layout. Now trying to record a ^ě shortcut will pass the validation, although the shortcuts have the same key code and modifiers and are in conflict. See #60 for previous discussion. No idea if we can handle this better, but at least we could describe the failing case in the spec and the docs.