Material input/output node (as in nodegroups)

This is something similar to some other proposal, but here I thought of a graphical and logical way to present the feature to the user, in a more consistent way.
The basic idea is to just follow the lines of how nodegroups work, considering materials as if they were "super nodes".
So the news here are:

• the "Material Input" node: as if it was an input group, it provides Shader, Volume Shader and Displacement data from material A, all to be used at will into material B, C etc... This would be so handy when you want to mix two or more already done/estabilished materials, into a new one. Or if you just want to build up on a material adding shaders, volume or even changing the displacement! No need to rearrange previous materials in nodegroups. No duplicated nodetrees. Just import all your material outputs into another one! (note: it shouldn't allow cross-reference between materials. Only linkable materials in the drop-down menu are shown)
  
  
• ability to plug any wire into the Output Material node (as in nodegroups): these are known as Arbitrary Output Values, and combined with the "Material Input" node above, they expand the concept: any data can be used in any other material B, C etc.., directly retrieving them from the original material A. A dedicated panel in the sidebar would let to rearrange and rename the AOVs, very similar to what we have in nodegroups.