Fix window thumbnails not working since frameworks 5.55

Summary:
This fixes a problem where window thumbnails are no longer visible since KDE Frameworks 5.55. This is caused by https://cgit.kde.org/plasma-framework.git/commit/?id=b257029afc3bfcd6366d2c2b910b8d91403ab8a which no longer automatically updates the visibility of mainItem of PlasmaCore.Dialog.

BUG: 404234

Test Plan: Ensure window thumbnails are working correctly now

Reviewers: mvourlakos

Reviewed By: mvourlakos

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19002