Show external suggestions based on both singular and plural text

[Данило Шеган]

We are currently showing external suggestions which have the same msgid (singular text), and we do not take msgid_plural (plural text) into account.

This behaviour is preserved with the refactoring, but we may want to change potmsgset._getExternalTranslationMessages to check both singular and plural when looking up suggestions. This would be more in line with current data model, especially if we change UI to allow selecting entire message (with all plural forms) at once, instead of selecting each plural form separately.

It was implemented and then removed in a revision with "timestamp: Fri 2007-11-16 18:29:09 +0100" by me, so maybe reusing that code might help.