When the previewed video stops playing (because of the user or because the media playback has ended), show the regular preview only if the preview setting is on.

Summary:
Bug result after test plan :

Test Plan:
In dolphin with the information panel
Select a video
Play the video
Disable the preview through the context menu (the video stays playing and visible and this will get fixed in a subsequent review)
Stop the video -> the video preview reappears when it should not

Reviewers: elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: elvisangelaccio, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D19843