Andrew Francis's Pageant updates: you can now multi-select key files

to add and keys to remove (in the open-file dialog and the key list
box respectively), and passphrases are cached during the process of
adding several keys at once (in case two of the keys have the same
passphrase).

git-svn-id: svn://svn.tartarus.org/sgt/putty@1466 cda61777-01e9-0310-a592-d414129be87e