[InformationPanel] Hide the video when the preview is disabled, avoid computing the preview when it is disabled

Summary:
Bug symptom : {F6698879}

Fix the bugs and allow to avoid launching a PreviewJob with all files even when the preview is disabled.

Test Plan:
 # In dolphin with an information panel with preview on
 # launch the video preview
 # Disable the preview

  ->bug 1: the video player control is still visible (this bug fix this)

 # re-enable preview

 -> bug 2: the video stays visible and the preview is displayed above (this bug fix this)

Reviewers: #dolphin, elvisangelaccio, ngraham

Reviewed By: #dolphin, elvisangelaccio, ngraham

Subscribers: ngraham, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19844