Tweak FF / REW / Next / Prev

FF is skip 10 sec, I think that's not enough. 30 sec is better for FF while 10 sec is good for REW IMHO.
Perhaps this should be a settings. I'd be more than willing to implement that if you like the idea.

Also, NEXT & PREV are not ideal IMO. PREV is unused (doubles as a REW button) and NEXT ends the video and (presumably) starts the next one in the queue. While I see the value in this, I think the inconsistency with the PREV button trumps this. Instead, Also, pretty much every other Media Player I use (Plex included) uses NEXT & PREV as a Large FF/REW action. Plex uses hard 10 min intervals now. It used to be +-10% which I liked much better so I set it up this way.
Again, I would gladly make this a setting if you feel that "NEXT means finish()" is still important.