Automatic housekeeping for "book id"

**Is your feature request related to a problem? Please describe.**
Currently the book id is more or less increased for every added book. If one deletes a book entry right in the middle of the library, there will be a gap in the series of book IDs.

**Describe the solution you'd like**
There should be some automatic "houskeeping" to fill that gaps. This might be triggered on a certain amount of library items maybe.