Port KeyboardLayout component to use generated DBus interface
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