Refactor GTK KCM code

Summary:
This diff introduces purely cosmetic changes.

1. KCM now compiled with no signals/slot keywords to avoid clash with gtk
2. Remove linking of XCursors library, because it is no longer needed
3. Replace some for-cycles with range-based ones
4. Wrap some string literals with QStringLiteral
5. Rearrange include headers
6. Some other changes to better match code style

Test Plan: Check, if there is no regressions

Reviewers: apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25872