KCM/Kwinoptions: Port focus tab to KConfigXT

Summary:
* remove singleton of KWinOptionsSettings

Depends on D27477

Test Plan:
* No functional change
* default / reset button work as expected.

Reviewers: ervin, bport, hchain, crossi, #kwin, zzag

Reviewed By: ervin, #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27934