Slideshow picture with video rework

This PR fixed some issues in slideshow/picture window:
1. unify slideshow/picture window behavior.
2. video in slideshow/picture nav now works as iDevice's photo app. when nav, show video thumb with a big play button.
3. better error handling, and skip error/unplayable picture during slideshow.

for touch screen, need this too: https://github.com/xbmc/skin.touched/pull/5