Possible redesign of the EntityTermsProvider interface

[thiemowmde]

This is an alternative proposal to #301.
#343 is a first step towards this.

• The idea is to refactor the EntityTerms class in a way that it represents 1 label, 1 description and 1 set of aliases that all have the same (base a.k.a. requested) language. Note that everything in that class should be represented as Term objects to support language fallbacks.
• AliasGroup will either be dropped in favor of EntityTerms or refactored to contain a list of Term objects, not a list of strings (this is a bit unrelated to this proposal, see Add requirements for language fall backs. #272 (comment) for the reasoning).
• The EntityTermsProvider interface will have a single method, getEntityTermsForLanguage( $languageCode ).
• TermList and AliasGroupList will be dropped.

This proposal will drop the possibility to get a list of all labels in all languages. This is on purpose since I think this use case is not relevant. If it turns out to be relevant we can add EntityTermsProvider::getEntityTerms, which returns an associative array of language code => EntityTerms objects.

[thiemowmde]

Moved to https://phabricator.wikimedia.org/T87237.