KeyManager: Change "About" string
Seems I wanted to fix #214 (closed) too quickly, and used "About Seahorse" instead of "About Passwords and Keys". I'm not sure who to exactly tag here, but hopefully translator-god @piotrdrag can tell me if it's okay to still merge this.
- Use a mnemonic (see #215 (closed))
- Don't use the actual app name (Seahorse) but the one that is shown to and known by the user (#214 (closed))