[3D] make "mono/2D" a default playback mode instead of a preferred stereo mode (it's no real stereomode)

Also removes the mode "none" from the list of preferred modes, which also doesn't make sense.
[3D] hide stereomode of movie in ask dialog if preferred mode is sames as movie mode
[3D] handle stereoscopic mode of videos in mixed playlists
[3D] show "same as movie" as preferred mode label in case of autodetection instead of the detected mode