#5 UI for signing keys

[hf]

Provide a UI for signing keys that doesn't fall back to
the command line. Would be more usable. :)

Inspiration for UI:

• with explicit selection of signing key (when you have
  more than one private key) and signature-checking-level
  as in Enigmail 0.89.5.0 OpenPGP Key Management
• with multiple key selection as in PGP 8.1

Applies to GPG Keychain Access 0.7.0PB3