multiview: start the new auto 3d mode
multiview: organize the wm_stereo_enabled flow

Is 3D enabled?
Y    |    N (2d)

Is 3D required?
Y     |   N (2d)

Is fullscren required?
Y     |   N (3d)

Are we on fullscreen?
Y (3d)|   N (2d + warning)

[ warning == 'double-click' or go to fullscreen to enable 3D ]
multiview: UI change, add 'Views' checkbox option to control view
the checkbox is not considered everywhere yet
multiview: integrate 'use_multiple_views' into the code