Add a commandline tool to switch lnf package

Summary:
using the kcm c++ itself to do the switch, have a tool
that can be used to switch lnf package from the command line

Test Plan: running the tool has the same effect of chosing a lnf trough the kcm

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9411