[sddm-theme] Use more precise labels for "Different User"

Summary: The "Different User" action button label is wrong. The respective button switches to a screen where usernames are manually inputted. The patch proposes a better label and also renames the label in this manual input screen.

Reviewers: ngraham, #plasma, #vdg, hein

Reviewed By: ngraham, #plasma, #vdg, hein

Subscribers: rooty, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18831