Summary:
QDBusInterface is useful for runtime introspection but little else.

As we know the interface in advance we can generate the keyboard layouts
interface and have non blocking, easier to read, easier to maintain
code.

Test Plan: Opened lock screen, it had the whole list, cycled between them

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22846