Add a QDialogButtonBox to the Coordinate Precision Dialog

Summary:
This was missing from the migration to Frameworks.

BUG: 414641

Test Plan: 1) Go to "Settings -> Set Coordinate Precision..." and change the coordinate precision

Reviewers: KDE Edu, apol

Reviewed By: apol

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D26166