Allow to skip episodes

See also https://seriesguide.uservoice.com/forums/189742-general/suggestions/3525612-allow-for-disabling-episodes

Sometimes you may not want to watch an episode (e.g. special, not available for streaming, disinterested), so you just want to skip it and continue with the following episode.

Also, allow skipping of whole seasons to e.g. start with a future season. Then maybe come back later to watch an older season.

Overview/Episode:
- [x] Skip button for button bar
- [ ] Season: display number of skipped episodes

Backend:
- [x] Switch watched binary flag (not watched/watched) to tri-flag (not watched/watched/skipped).
- [x] Rewrite any code that assumes it as binary, create `EpisodeFlag.FLAG` constants and `EpisodeTools.isWatched(flag)` and similar for easy use.

Resolved:
- Episode Sort Unwatched First: keep, unwatched sorted first, then watched, then skipped
- Data liberation: updated, converts flag into watched and skipped boolean property
- DBUtils next episode algorithm: already works on int, should work fine
- DBUtils season counter algorithm: also works on int, works fine. Should be adapted to also display skipped episodes.