Touchpad KCM Pointer Speed Slider Improvement

Summary:
Slider value to libinput value mapping algorithm is changed. Tested and everything is working fine.
You can test it by changing value on slider and checking the libinput value from ~/.config/kcminputrc
UI is same so I don't share screenshot.

Reviewers: ngraham, romangg, Plasma

Reviewed By: ngraham, romangg, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D13767