Add a fancier authentication dialog (Mac and Linux).

Add a new authentication dialog. The current Mac one is based on applescript, and while it's functional, it's a bit ugly, and might misbehave if the user inputs an incorrect password. This new one will have a nicer look and feel, and will also manage incorrect passwords better.

This was planned to be only for the Mac, but seeing as it's a cross-compatible front-end to sudo, I'll use it for both Linux and Mac OS X.