[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:
{F6503161}

After:
{F6503163}

Reviewers: #kwin, #vdg, graesslin

Reviewed By: #kwin, graesslin

Subscribers: kwin

Tags: #kwin

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