Configuration to never show the passwors dialog

Summary:
on Plasma mobile, not qwidget dialogs should be ever shown at any time.
the networkmanager KCM includes its own password field, so the password dialog can be safely
be never shown

Test Plan: fails silently

Reviewers: #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18588