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