Handle invalid identifiers when scrobbling.
Use X-Plex-Platform instead of X-Plex-Client-Platform.
Improve direct play -> transcode fallback.

If we were already transcoding, whether it was a remux or transcode, don't
fall back to anything. This was causing users to report that part way through
videos would restart from the beginning. What was actually happening is that
the transcoder was dying, but then we tried the same thing again and it would
fail in the same place.

And if we fall back after having successfully played some amount of the video,
restart from that offset rather than the original seek value.
Swap the play/resume order and prompt more often. Fixes #185. Fixes #188.
Use our packaged section icons instead of the default PMS ones, #190.

Until we can ask PMS not to use transparency, just ignore the PMS section
icons if they aren't pointing to custom paths.