gnome3: Convert password/confirmation to asynchronous model.

* gnome3/pinentry-gnome3.c (gnome3_cmd_handler): Convert main part of
password or confirmation fetching into asynchronous code by moving
completion into...
(_gcr_prompt_password_done): ... here and...
(_gcr_prompt_confirm_done): ... here.

The async programming interface to gcr is necessary if we want to be
able to enforce a timeout, which will happen in the next patch in this
series.

Did not apply cleanluy due to me comment reformatting in a former
patch. Fixed. -wk

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>