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! ;-)
Did you read the source code of `DocSettings`? The open method of `DocSettings` accept the file path of the document not the history file itself. `DocSettings` knows where the history file is stored.  The benefit to use the well predefined interface of `DocSettings` is that
1. you can write less code 
2. we can change the behavior for example the location of the history files with minimum code change.