Resuming playback on movie extras

Currently PMS does not keep track of playback of movie extras, whether they are from online sources or locally added. I would definitely like to have playback supported for extras, at least for locally added extras.

I can resume extras, at least for locally stored ones on Android, PHT, and PMP. I’m not sure about online sources since none of them seem long enough for it to matter anyways.

2021 clean-up: implemented