Use GNOME GnuPG agent

It would be overall nicer (and safer when using GNOME Shell, due to the way the passphrase window takes over the desktop in ways that no other process can fake) to use the GNOME GnuPG agent instead of the regular one.
Moreover, it’s likely that i18n bugs such as #6206 (closed) don’t affect the GNOME agent.

For the record, we had two reasons to move to gpg-agent + pinentry (#5419 (closed)):

the dreaded desktop lock bug (#5586 (closed)), which is fixed on Wheezy
OpenPGP smartcards support (#5931 (closed))

Subtasks

#6272 (closed)
#11637 (closed)

Related issues

Blocked by #6015 (closed)
Blocked by #5586 (closed)

Original created by @intrigeri on 6271 (Redmine)

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