kcms/{mouse,touchpad}: Use checkbox for pointer acceleration
Instead of two radio selector options. There's little point in having a "None" option when the other option is essentially "yes, enable this".
Even with upcoming custom acceleration curves, this will still make sense because pointer acceleration is enabled both for "Adaptive" and "Custom" modes, just with different acceleration curves.
Screenshots
Mouse KCM is also changed, looks the same as Touchpad KCM depicted here, except with fewer options overall.
| Before | After |
|---|---|
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Curiously, Kirigami.ContextualHelpButton's text pop-up doesn't have a drop shadow, unlike the regular QQC2.ToolTip.
Test plan
Disable and Apply. Move from the left end of your touchpad to the right end, and back, a few times. Always end up at roughly the same spot.
Enable and Apply. Move fast and you've way overshot compared to before.

