[LNF KCM] make it possible to close the preview

Summary:
The preview window doesn't respond to setting its visibility to false, so currently
it can't actually be closed without using the task manager or another external way
of closing windows.

Fix the issue by triggering the window's close() function instead.

Test Plan: Preview a theme and click on the close button, or anywhere else, or hit the Esc key

Reviewers: Plasma, apol, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: ognarb, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22543