Change krunner calculator to display significant digits

Summary:
Change the qalculate krunner module to print only significant digits instead of an interval as this is sometimes unreadable.
BUG: 412627
FIXED-IN: 5.18.0

Test Plan:
Before this change:
{F7828390}
After:
{F7828389}

Reviewers: broulik, #plasma

Reviewed By: broulik, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26149