Exclude a book cover from Kobo's screensaver

For books without real or with a bad cover, or books you don't want others to see that you are reading them.

No global variables were created with this CR! ;-)
Thanks, that did it!
And yes, i read it, but somehow I missed this. Although it really is quite obvious now that I know it, but even reading it a few times I did not even think of this being possible...
I'll close this one here and open a new PR.