authPrompt: add support for auth without username

This commit introduces a new BeginRequestType enum which gets
passed to the 'reset' signal to specify whether
a username should be provided to the begin() method and changes
the loginDialog to comply.

Currently, the signal only ever gets emitted with

AuthPrompt.BeginRequestType.PROVIDE_USERNAME

but that will change in the future when providing smartcard
support.

https://bugzilla.gnome.org/show_bug.cgi?id=683437