Add "translation memory"

[humdingerb]

There may be other, official definitions of "translation memory". I mean that, independent of project, once an English string was translated and saved, that English string and its translations are put in a database. Whenenver that exact English string appears in any project, it's translation is offered as a suggestion.

That's, come to think of it, similar to the "Glossary" we already have, though that one is user-managed.

Is that feasible, or would it get too big and slow?

Anyway, this would make the issue described in #74 less of a hassle. You'd just have to go through the suggestions for the strings that are unchanged, but got an additional comment or changed context.