Port kcm input to UI File.
Summary:
A split version of D8168. This only refactors the hardcoded ui constructor
into an equilvalent UI file. This change has no string change nor behavior
change.
Test Plan: Test config opton manually.
Reviewers: subdiff, davidedmundson, ngraham
Reviewed By: subdiff
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D8456