Refactor language chooser to display as a menu

- use `dl. actionMenu` HTML markup for language switcher, showing the current language
- make language switcher available on site root too, if subsites have `linkSiteInLanguagechooser` enabled
- add a `ILanguages` adapter for easier finding related languages
- refactor switcher code and tests

@ninfaj could you take a look at this one?
Should we include some default CSS in order to style the language chooser for default Plone?

/cc @maethu