Allow to add/assign custom materials per mesh after file load

[Colorbleed]

By default Substance Painter only allows to have materials that are present e.g. in the source FBX file. It would be great however if there were a way to also allow the user to assign a new/custom material to specific meshes so that if e.g. an Alembic is loaded with multiple meshes and there only being a single "DefaultMaterial" entry that the user has the ability to split it into multiple materials directly in Substance Painter.