Library : Filter only merged books

[N3OP0L1T]

• I would like to request a filter to sort only merged books.

~One more, ability to rename the chapters of the book made manually~ => Done in #859

Hope you find these sufficient enough changes to implement.

[RobbWatershed]

Rename chapters has been done there https://github.com/avluis/Hentoid/issues/859 and is available on today's v1.19.7

Filter to only merged books sounds legit. Thanks for your suggestion.