Detect changed source string in bilingual XLIFF

With xliff it's a bit different - the strings are identified by IDs, so people tend to expect Weblate to do this kind of thing, but it behaves same as with other formats. The easy fix for this is to configure the translation as monolingual, it will then work as expected.

_Originally posted by @nijel in https://github.com/WeblateOrg/weblate/issues/3330#issuecomment-568038543_