[effects/thumbnailaside] Save shortcuts

Summary:
If user selects a shortcut to trigger the effect, that shortcut won't
work after the KCM is closed because we don't save it.

BUG: 400351
FIXED-IN: 5.14.3

Test Plan:

• Specified custom shortcut to trigger the effect;
• Closed the KCM;
• Pressed the shortcut, a thumbnail appeared.

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16456