Changed index building to take chapters into account

Andreas Hellquist requested to merge andreas/zathura-pdf-mupdf:develop into develop

In mupdf, documents are divided into chapters and pages rather than just pages. This lead to a bug where index mode would link to sections without taking chapters into consideration, meaning that every chapter would link to page 1 (as described in this issue).

This merge requests solves the problem by taking the chapter into account when generating the index.

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