[3D] Add stereomode of videos to streamdetails and expose info to skins

This PR extends the streamdetails (codec, width, height, ...) with a new field for "stereoMode" and makes this info available to skins via ListItem.StereoscopicMode. In addition it adds "3D" flags to the various list views and video info dialog. In case a movie/video is 3D, the 3D icon will be shown instead of SD/HD to not clutter the GUI any further (there are no SD 3D movies anyways).

The skin changes depend on #3568 to work properly, so ideally that PR should be merged first.