Cutting Edge: Improved Authentication

As the authentication code and dialog box were modal, no "check for updates" could be performed before Google authentication.

The authentication part of Mailplane is now completely new: Mailplane will automatically check for updates, even if you can't sign in.
The dialog is now a "dialog sheet" attached to the main window, and the login procedure can be cancelled at any time.

Plus: If you have many accounts, startup is now much faster.