enhance Photo view

- photo transcoder setting to force transcoding (eg. for EXIF rotation)
- fix aTV caching issue (it wouldn’t load another, higher-res
transcoded image, leaving the pics rather pixelated)
fix reported aTV IP in "forwarding" environments (X-Forward-For", eg. on Synology)
add support for audio transcoder
enable photo channels

- tested with TheBigPicture, PhotoShow, NYTimesLens
- not sure about the SlideShow…
rename MEDIAURL->VIDEOURL (compare image-, musicurl)
Merge remote-tracking branch 'upstream/master'

Conflicts:
	XMLConverter.py