Add Episode Playback Progress

This adds a playback progress bar to the episode preplay screen on iOS 6.0+

The bars match the style that seemed most popular on the forums...
https://forums.plexapp.com/index.php/topic/88102-episode-play-progress/?p=514566

There is an empty bar on unwatched content, a partially full bar on partially watched content, and no bar on fully viewed content. No blue dots at all.