Python API: add Python-defined node groups for shaders and compositing.

This was already supported for Cycles shader nodes, but now also works for
Eevee and compositing nodes. Instead of a generic NodeCustomGroup, now
there is ShaderNodeCustomGroup and CompositorNodeCustomGroup that can be
subclassed and registered.

Differential Revision: https://developer.blender.org/D4370

release/scripts/startup/nodeitems_builtins.py

@@ -99,7 +99,9 @@ def node_group_items(context):
         # filter out recursive groups
         if contains_group(group, ntree):
             continue
-
+        # filter out hidden nodetrees
+        if group.name.startswith('.'):
+            continue
         yield NodeItem(node_tree_group_type[group.bl_idname],
                        group.name,
                        {"node_tree": "bpy.data.node_groups[%r]" % group.name})