[KCMs] Make spinboxes editable

Summary:
Otherwise they're annoying to use because you can only interact with them using a scroll
or by clicking on the tiny arrows.

Test Plan: You can now enter text in the spinboxes

Reviewers: #vdg, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24814