Refactor kcm_input to enable having multiple backends.

Summary:
Split patch for D8168. This change only refactor the X11-related code into
its own places. KCM itself will only use the backend interface.

Test Plan: Manually test all options.

Reviewers: subdiff, davidedmundson, ngraham, #plasma

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8460