PuTTYgen: add an extra button to save a public key into a file

PuTTYgen: add an extra button to save a public key into a file
(as well as showing it for cut and paste). For SSH1, this feature is
largely cosmetic and added for orthogonality; it comes into its own
in SSH2, where it saves the Official One True Public Key Format as
specified in the draft spec, and more particularly as used by
ssh.com's product for authentication. Now that ssh-3.0.1 supports
RSA user keys, this is suddenly actually useful.

[originally from svn r1217]