feat(kcm): always use 0.25 slider scale steps

Summary:
The number of sllider ticks is reduced, making the slider more easy to control.
Fine-grained control is still possible via the spin box.

This way we have the same behavior on X11 and Wayland.

Test Plan: Manually

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T11928

Differential Revision: https://phabricator.kde.org/D25026