Fix confusing tooltip strings

Based on user feedback:
- Avoids the use of validity for the lifespan of the certificate
(User feeback was -> Kleopatra says it's valid but a verification
is not green)
- Include UserID validity in tooltip.
- Avoid the word certificate.
- Shorten and clarify usage.

This also should show the certification status more prominent
as this is a use case of OpenPGP usage for Kleopatra.