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

What title says. This PR extends the streamdetails (codec, width, height, ...) with a new field for "stereoMode" and makes this info available to skins via ListItem.StereoscopicMode and ListItem.IsStereoscopic. 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).

This PR is RFC for now because there are some things I'd like to discuss/resolve first.