Allow single images to be excluded from the slideshow

Summary:
This allows to exlude single images from the slideshow by unchecking the little checkbox in the top right corner.
Feature or todo?: If a folder is removed I don't touch the unchecked Slides. So the list could get very big but if a formerly removed folder is
added again an excluded image will be excluded again.

Test Plan:
{F6684113}
- Uncheck some images
- They don't appear in the slideshow

Reviewers: #plasma, #vdg, ngraham, davidedmundson

Reviewed By: #plasma, #vdg, ngraham, davidedmundson

Subscribers: davidedmundson, filipf, abetts, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19687