Add methods to hide the reveal password action

The new `lineedit_reveal_password` kiosk key cannot be queried from
kwidgetsaddons (being tier1), but we can at least add methods to let
client manually disable the reveal action, if necessary.

Reviewers: cfeck, broulik

Subscribers: #frameworks

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D4355

Closes T5033