lupdate: translations from different contexts not recycled

When messages are moved from one context to another, they should be re-used, provided some conditions are fulfilled:

• if a context (class) is simply renamed, all translations should be re-used and marked as finished
• if a sufficiently unique message (whole sentence or at least three words) is moved to a different context, it should be re-used and marked as finished
• messages with less than three words should be re-used, but not marked as finished