Even though having a sub-menu named Stereoscopy improves
discoverability, it also ends up making layout convoluted
for a single item.
There are no plans on extending stereo nodes. Users who
want this likely already make their own node group
setups (and in the future, catalogs).
Based on a discussion with Sergey.
* Move `Bokeh Image` and `Texture` back to `Input`
* Move `Mix` to the top level
* Move `Posterize` to `Filter`
* Move `Set Alpha` to `Color`
* Move `Color Spill` to `Keying`
* Rename `Mix` node label to `Mix Color`
Pablo Vazquez