Allow typing in font DPI spinbox

Summary:
Otherwise keyboard use is hard, this fixes a regression that we could no
longer select DPIs that weren't intervals of 24

Test Plan: Ran, typed

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D13050