[Feature request] Encrypt saved bookmarks with master password or keyring-saved key

[tjohnman]

In order to be able to use gFTP conveniently with private servers, there needs to be a way to store passwords securely. Right now, passwords in bookmarks are saved scrambled (gftp_scramble_password). There should be an option to use a master password, or to use the user's keyring, to encrypt them.