Show more information on CUPS auth dialog

Summary:
This extends 'KCupsPasswordDialog' so that a prompt text can be
set.
It is explicitly stated that authentication is required
by CUPS, which was not totally clear beforehand and
prompt information as passed via the CUPS
callback function is displayed in addition.

This e.g. makes clear what host the authentication
information is sent to.

BUGS: 397125

Test Plan:
Set up a remote CUPS server that requires authentication
and modify '~/.cups/client.conf' so that it points to this server,
e.g. as described in https://bugs.kde.org/show_bug.cgi?id=397125#c6 .

Start print-manager and make sure the authentication dialog is clear
what the authentication is needed for and where the information is sent.

Reviewers: dantti, ngraham

Reviewed By: dantti, ngraham

Subscribers: ltoscano, ngraham, kde-utils-devel

Differential Revision: https://phabricator.kde.org/D18648