Provide a way for developers to easily set up plasma sessions for their development environment

Summary: Creates the necessary files in the build directory as well as provide a script that will install them to /usr so the login manager can find it.

Reviewers: ngraham, #plasma, hein

Reviewed By: ngraham, #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21992