Show 3 significant figures when displaying doubles

Summary:
Show 3 significant digits for double values when
displaying them with e.g. baloo-widgets
Also fixes localization
CCBUG: 343273

Reviewers: ngraham, bruns

Reviewed By: bruns

Subscribers: kde-frameworks-devel, #baloo

Tags: #frameworks, #baloo

Differential Revision: https://phabricator.kde.org/D21499