xkbcomp: Enable lists of keysyms and actions {} and {a} Motivations: - Follow the principle of least astonishment; - Ensure consistency; - Enhance the use of custom defaults; - Facilitate the tests. There is some ambiguity because we use `{}` to denote both an empty list of keysyms and an empty list of actions. But as soon as we get a keysym or an action, we know whether it is a `MultiKeySymList` or a `MultiActionList`. So we just count the `{}` at the *beginning* using `NoSymbolOrActionList`, then replace it by the relevant count of `NoSymbol` or `NoAction()` once the ambiguity is solved. If not, this is a list of empties of *some* type: we drop those empties and delegate the type resolution using `ExprEmptyList()`.