[WIP] major material rework

[Tarcontar]

Got most of the stuff for import working so far. Export still needs some work. Encountered some issues with missing functionality in the blender python api in order to complete this:

• can not set 'default_value' for custom node sockets since no dedicated interface can be defined for them (is always NodeSocketInterface which does not provide a default_value)

This way stuff like the alpha pipeline can not be provided as node group templates since a user would have to fill in the default values for math nodes himself which is not feasable.

• can not set the 'type' of custom node sockets thus they always default to 'VALUE' which causes e.g. ColorSockets to provide only the red channel as output

[Tarcontar]

Found a solution for the NodeSocketInterface problem as described here: https://developer.blender.org/T78075

[Tarcontar]

replaced by PR #170