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