This patch implements the list panel system D7490 for grease pencil
shader effects. It also moves their drawing to a callback in
ShaderFxTypeInfo in line with the extensible architecture refactoring
goal T75724.
The implementation is basically exactly the same as for the modifier
patch (9b099c8612).
Thanks to Matias Mendiola (@mendio) for helping to develop the layout
changes.
Differential Revision: https://developer.blender.org/D7985
