NEW Merge i18nTextCollector with existing (fixes #1838)

This is a necessity for any further 3.1 pushes of master files to getlocalization.
Because we'd otherwise remove existing master strings for CTF etc,
which means we can no longer backport new translations to 3.0
(and there's no way for users to contribute translations to 3.0 via getlocalization).

It's still a very monolithic class, but at least I've refactored it to return
all collected strings without writing it to files (for easier testing).