Implement the DBus org.kde.KeyboardLayouts interface of keyboard kded

Summary:
This change introduces a new class KeyboardLayoutDBusInterface which
implements the same DBus interface as the keyboard kded module.

Thus components which interact with the keyboard kded through dbus start
to also work on Wayland.

Together with D4322 this should result in keyboard layout being available
on the lock screen.

T5209

Test Plan:
Tested with qdbusviewer: switching layout works, signal on
change gets emitted.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D4323