Change kcm header "Select Login Theme" to "Configure the SDDM Login Manager"

Summary:
Change kcm header "Select Login Theme" to "Configure the SDDM Login
Manager" because it's not only about the theme, you can also setup
autologin and set the halt and reboot commands.

Reviewers: plasma-devel, davidedmundson, ngraham

Reviewed By: ngraham

Subscribers: ngraham, davidedmundson

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8265