Cleanup: Nodes: Store node group idname in tree type

There was already a utility to retrieve the correct node group idname
from the context, `node_group_idname`, but often it's clearer to
use lower-level arguments, or the context isn't accessible.
Storing the group idname in the tree type makes it accessible
without rewriting it elsewhere.

source/blender/blenkernel/BKE_node.h

@@ -374,6 +374,9 @@ typedef struct bNodeTreeType {
   int type;        /* type identifier */
   char idname[64]; /* identifier name */
 
+  /* The ID name of group nodes for this type. */
+  char group_idname[64];
+
   char ui_name[64];
   char ui_description[256];
   int ui_icon;

source/blender/editors/space_node/node_group.cc

@@ -44,7 +44,12 @@
 #include "UI_resources.h"
 
 #include "NOD_common.h"
+#include "NOD_composite.h"
+#include "NOD_geometry.h"
+#include "NOD_shader.h"
 #include "NOD_socket.h"
+#include "NOD_texture.h"
+
 #include "node_intern.hh" /* own include */
 
 namespace blender::ed::space_node {
@@ -99,16 +104,16 @@ const char *node_group_idname(bContext *C)
   SpaceNode *snode = CTX_wm_space_node(C);
 
   if (ED_node_is_shader(snode)) {
-    return "ShaderNodeGroup";
+    return ntreeType_Shader->group_idname;
   }
   if (ED_node_is_compositor(snode)) {
-    return "CompositorNodeGroup";
+    return ntreeType_Composite->group_idname;
   }
   if (ED_node_is_texture(snode)) {
-    return "TextureNodeGroup";
+    return ntreeType_Texture->group_idname;
   }
   if (ED_node_is_geometry(snode)) {
-    return "GeometryNodeGroup";
+    return ntreeType_Geometry->group_idname;
   }
 
   return "";

source/blender/nodes/composite/node_composite_tree.cc

@@ -183,6 +183,7 @@ void register_node_tree_type_cmp()
 
   tt->type = NTREE_COMPOSIT;
   strcpy(tt->idname, "CompositorNodeTree");
+  strcpy(tt->group_idname, "CompositorNodeGroup");
   strcpy(tt->ui_name, N_("Compositor"));
   tt->ui_icon = ICON_NODE_COMPOSITING;
   strcpy(tt->ui_description, N_("Compositing nodes"));

source/blender/nodes/geometry/node_geometry_tree.cc

@@ -109,6 +109,7 @@ void register_node_tree_type_geo()
       MEM_callocN(sizeof(bNodeTreeType), "geometry node tree type"));
   tt->type = NTREE_GEOMETRY;
   strcpy(tt->idname, "GeometryNodeTree");
+  strcpy(tt->group_idname, "GeometryNodeGroup");
   strcpy(tt->ui_name, N_("Geometry Node Editor"));
   tt->ui_icon = ICON_GEOMETRY_NODES;
   strcpy(tt->ui_description, N_("Geometry nodes"));

source/blender/nodes/shader/node_shader_tree.cc

@@ -166,6 +166,7 @@ void register_node_tree_type_sh()
 
   tt->type = NTREE_SHADER;
   strcpy(tt->idname, "ShaderNodeTree");
+  strcpy(tt->group_idname, "ShaderNodeGroup");
   strcpy(tt->ui_name, N_("Shader Editor"));
   tt->ui_icon = ICON_NODE_MATERIAL;
   strcpy(tt->ui_description, N_("Shader nodes"));

source/blender/nodes/texture/node_texture_tree.c

@@ -140,6 +140,7 @@ void register_node_tree_type_tex(void)
 
   tt->type = NTREE_TEXTURE;
   strcpy(tt->idname, "TextureNodeTree");
+  strcpy(tt->group_idname, "TextureNodeGroup");
   strcpy(tt->ui_name, N_("Texture Node Editor"));
   tt->ui_icon = ICON_NODE_TEXTURE; /* Defined in `drawnode.c`. */
   strcpy(tt->ui_description, N_("Texture nodes"));