Add StatementListChanger

[thiemowmde]

• This wraps the last remaining usage of the ByPropertyIdArray class. This is meant as a migration path. First, all relevant tests from ByPropertyIdArrayTest should be carried over. Then the implementation can be changed to not rely on ByPropertyIdArray any more. ByPropertyIdArray can be removed then.
• I decided to not reimplement certain features of ByPropertyIdArray, especially it's magic behavior of reordering groups.
• This also adds a few service methods that are already in use in different places in the Wikibase.git code base, but with individual private implementations.