Thomas Carmichael authored May 26, 2018 and Elvis Angelaccio committed May 26, 2018

Summary:
By default, a filmstrip effect is embedded in the thumbnail that is generated by ffmpegthumbs.

This adds the capability to configure the thumbnailer so as not to embed the filmstrip effect, if so desired.

Test Plan:
 - Configure the ffmpegthumbs thumbnailer through the Dolphin Preferences > General > Previews tab
 - Un-tick the default option to embed the filmstrip effect and 'Ok' both preferences windows
 - Enter into a folder or refresh a folder with video files to confirm that video thumbnails no longer have the filmstrip effect embedded

Reviewers: #dolphin, ngraham, elvisangelaccio

Reviewed By: #dolphin, ngraham, elvisangelaccio

Subscribers: elvisangelaccio, cfeck

Differential Revision: https://phabricator.kde.org/D12918