This fixes a number of issues.
Replaced ffmpeg/ffprobe with LGPL versions for #384.
Fixed #387 to not lock up "Show File" for some linux users.
Split and join audio files have been revamped. Joining now asks you to select a directory that contains audio files. Cover art is used.
Fixed #392 where not getting artwork might cause a split to not finish
Fixed #403 to allow splitting multiple books at once.
Added "Book Report" action item to get detailed report about a book. (for debugging, but might be helpful.)
Revamped book file cache for improved performance on larger libraries.