feat(kcm): always use 0.25 slider scale steps
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
Maniphest Tasks: T11928
Differential Revision: https://phabricator.kde.org/D25026