multiview: viewport ui changes + camera ui fix

related to #66
Still need to use the viewport camera_stereoscopy selection in the code
multiview: remove custom suffix and STEREO_CENTER_IDd
multiview: viewport further changes, getting the right 'basic stereo'
camera and placeholder code for the multiviews camera
close #66
multiview: stereo_camera no longer needed (remove DNA)
multiview: fix/change: No longer to have a "per-window" 3D toggle
close #63
multiview: removing SceneRenderView->camera
Note: rendering is not working at the moment:

for basic stereo we need to find the place where the matrices are used
in the renderer, and provide the renderer with the calculated ones (we
may need/should store them in the RenderView)

for advanced stereo we need to write the actual code for BKE_camera_multiview_advanced
(this should be easy)

@Eibriel, if you can look at the above it would be great. I have no idea
how difficult it will be. (I can do the advanced one tomorrow-ish)