3d: simplify stereoscopic system settings to only include one mode

There now only exist a preferred mode, which gui switch to on demand. During playback it's possible to change modes and we will return to normal mode when playback completes. If keymaps are used with no playback, we switch system settings based on the action, but ask before.
fixup! dvdplayer: pass stereo_mode to renderers and select what to render
WIP: allow setting the video's stereo scopic mode from video settings