Highlighted atoms in Material visualizer

@himanel1 @ndaelman I was thinking that it will be a very nice feature to have a "highlighted atoms" section that, in the structure visualizer works in a similar way as the topology cases: i.e., some sub-structure of highlighted atoms that when clicked will make all the non-highlighted more transparent.

I will be happy to implement this, as part of the Projection (we can parse the only atoms used for the projection to highlight them) and Core-Hole (only certain atoms in the supercell contains holes in the core orbitals) metainfo. The later is related with #1316 (closed) and #1371 (closed).

@himanel1 : where are the modules defining all these plotting? Maybe you can quickly point me in the correct direction.