This commit adds the rest of the support for actually being able to use
Overdrive MediaMarkers.
Shoutout to benonymity's project OverdriveChapterize, where I was able
to port over the logic to actually do the timestamp conversions
https://github.com/benonymity/OverdriveChapterizer/blob/main/chapters.py
I still need to do a lot of cleanup of the actual code, and finish the UI.