Kill Runner: Move config keys to new file

Summary:
The config keys/sorting enum have been moved to a new file, before this the runner included the header of the config class.
This patch originated from https://phabricator.kde.org/D27513, but it was requested to split it up into separate patches.

Test Plan: Should compile.

Reviewers: davidedmundson, ngraham, broulik, meven, apol

Reviewed By: meven

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27576