Fix HSV/HSL filter widget slider management

Summary:
Fix for https://bugs.kde.org/show_bug.cgi?id=363882 and related issues in the HSV/HSL filter dialog.

Moves the fragile conditionals to a more data driven model.

Reviewers: dkazakov, Krita, rempt

Reviewed By: Krita, rempt

Subscribers: rempt, dkazakov, woltherav

Differential Revision: https://phabricator.kde.org/D2115