Add variable file import related method to listener version 3

[pekkaklarck]

Listener version 2 has a hook method that's called when a variable file is imported. We should add such a method also to the version 3 API.

Related library and resource file import methods are added already as part of RF 7.1 (#5008). The reason the variable file method isn't added is that there's no suitable model object representing variable files available. Such a model object is likely to be added in the future when we also otherwise enhance variables and variable namespaces. After that adding the method itself is easy.