make text of label of password input dialog for GnuPG keys copyable

When `seahorse` presents the dialog for entering the password for a GnuPG key and offers to store it in a keyring, it'd be nice to be able to copy details of the text label.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: seahorse 3.15.90-0ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-11.11-generic 3.19.3
Uname: Linux 3.19.0-11-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.17-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Apr 3 13:47:42 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (0 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: seahorse
UpgradeStatus: No upgrade log present (probably fresh install)