Improve pps_document_fonts_scan API

The "return" value means only whether fonts were found. This is a bug from previously in Evince and Poppler[1]. So don't return anything, and instead always scan the whole document, as that's how it makes most sense to use it. This allows to keep all the logic more contained in pdf_document_fonts_scan, and to require less boilerplate around.

[1] https://gitlab.freedesktop.org/poppler/poppler/-/merge_requests/1487

And fix a bit the shell part

This fixes the crashes when opening the dialog multiple times.

Helps #26

Fixes: #29 (closed)