use dbus interface to switch shell l&f

Summary:
the shell has 2 methods to switch lnf layout:
one that throws away the config contents, one that
keeps them

Reviewers: davidedmundson, Plasma

Reviewed By: davidedmundson, Plasma

Subscribers: colomar, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2448