[kcmkwin/desktop] Disable the OSD duration spinbox if the corresponding checkbox is unchecked

Summary:
In the old version of the KCM, one could change value of the spinbox
only if "Desktop Switch On-Screen Display" is checked.

Test Plan:
Before:

After:

Reviewers: KWin, VDG, graesslin

Reviewed By: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17754