[3D] usability fixes

This PR fixes several usability issues with 3D playback handling:
* moves the "2D" stereoscopic mode from "Settings->System->Video->Preferred stereo mode" to "Settings->Video->Playback->Preferred stereoscopic playback mode" because 2D is no 3D mode.
This fixes the issue that you couldn't use the 2D->3D toggle action/buttion if you preferred playback to be started in 2D and wanted to switch later to your preferred 3D mode, because preferred 3D mode had to be set to 2D in order to get 2D playback by default
* makes "autodetect / same as movie" the default "preferred stereomode" and removes the "none" option from the list of preferred modes (a preferred mode has to be a valid mode and not NONE)
* removes the select option of the movies 3D mode in the ask dialog in case the preferred mode was set to auto (which also means it's using the movies mode - thus option was superfluos in those cases)
* changes the label of the "preferred stereo mode" option in the ask dialog to "same as movie" in case that's the preferred mode. Before we resolved the resulting preferred mode and show "side by side" or "top bottom", but this is IMO misleading as users confirmed
* removes a duplicate label

By removing the "NONE" option from the preferred stereomode setting, it's now no longer possible to disable stereoscopic handling entirely in case we detected a 3D movie. If we still want the option to turn of stereoscopic handling for video playback, I can add a 4th option "none" to the "preferred playback mode" setting.

@elupus please review
@jmarshallnz - would also apply to Gotham if you consider these as relevant