gdm: disallow cancel after verification succeeds

Once verification has succeeded, the train's already
left the building and we shouldn't allow canceling.

This commit renders the cancel button non-reactive
and makes the cancel function be a noop after
verification succeeds.

https://bugzilla.gnome.org/show_bug.cgi?id=740141
gdm: fix sensitivity of auth prompt when cancelling early and user list is disabled

If the user list is disabled and the user clicks cancel quickly enough
after typing their username, they can get in a state where the
auth prompt gets stuck in the insensitive state.

This is because the login dialog code makes the prompt insensitive
while while pam is processing the provided username, but the prompt
only makes itself sensitive again when it is hidden.

This commit makes it sensitive right before asking for a username again.

https://bugzilla.gnome.org/show_bug.cgi?id=740141