Small UI fix for magic wand tool

Summary:
Made small UI fix for "Contiguous Area Selection tool" (or "magic wand tool") as suggested in this theme (first part): https://forum.kde.org/viewtopic.php?f=288&t=140179

Changed "fuziness" slider value from 0-200 to 1-100. I think it don't make sense to use 0 (because it doesn't select anything) or 100+ (because it select everything)
Also changed slider step from 10 to 1 and made it exponential (1-20 values is most common)

The patch by Radian <xenys25@gmail.com>

Reviewers: Krita, rempt, dkazakov
Reviewed By: Krita, dkazakov
Differential Revision: https://phabricator.kde.org/D5793