Make it possible to turn off shortcut validation
Make it possible to turn off shortcut validation

Previously, setting a nil `shortcutValidator` on `MASShortcutView` would
lead to all shortcuts considered invalid. I think it’s better the other
way around, where setting “no validator” means the caller wants no validation
done at all.

(I actually have a use case for this. Under normal circumstances, recording
keys with no validation doesn’t have much sense, but I use `MASShortcutView`
to record shortcuts that will be used in a different context, in the browser.)

shpakovski/MASShortcut,shpakovski/MASShortcut,shpakovski/MASShortcut