Geometry Nodes: unify icons -> use ICON_NODETREE for everything

Until there is a icon made specially for this, the nodetree icon is up
for grabs. Using it in the nodegroup + modifier + editor helps the users
to make a connection on where to edit those modifiers.

source/blender/editors/space_node/drawnode.c

@@ -3357,7 +3357,7 @@ void ED_node_init_butfuncs(void)
   ntreeType_Composite->ui_icon = ICON_NODE_COMPOSITING;
   ntreeType_Shader->ui_icon = ICON_NODE_MATERIAL;
   ntreeType_Texture->ui_icon = ICON_NODE_TEXTURE;
-  ntreeType_Geometry->ui_icon = ICON_PHYSICS; /* TODO: Use correct icon. */
+  ntreeType_Geometry->ui_icon = ICON_NODETREE;
 }
 
 void ED_init_custom_node_type(bNodeType *ntype)

source/blender/makesrna/intern/rna_modifier.c

@@ -6972,7 +6972,7 @@ static void rna_def_modifier_nodes(BlenderRNA *brna)
   srna = RNA_def_struct(brna, "NodesModifier", "Modifier");
   RNA_def_struct_ui_text(srna, "Nodes Modifier", "");
   RNA_def_struct_sdna(srna, "NodesModifierData");
-  RNA_def_struct_ui_icon(srna, ICON_MESH_DATA); /* TODO: Use correct icon. */
+  RNA_def_struct_ui_icon(srna, ICON_NODETREE);
 
   RNA_define_lib_overridable(true);
 

source/blender/makesrna/intern/rna_nodetree.c

@@ -9851,7 +9851,7 @@ static void rna_def_nodetree(BlenderRNA *brna)
       {NTREE_SHADER, "SHADER", ICON_MATERIAL, "Shader", "Shader nodes"},
       {NTREE_TEXTURE, "TEXTURE", ICON_TEXTURE, "Texture", "Texture nodes"},
       {NTREE_COMPOSIT, "COMPOSITING", ICON_RENDERLAYERS, "Compositing", "Compositing nodes"},
-      {NTREE_GEOMETRY, "GEOMETRY", ICON_MESH_DATA, "Geometry", "Geometry nodes"},
+      {NTREE_GEOMETRY, "GEOMETRY", ICON_NODETREE, "Geometry", "Geometry nodes"},
       {0, NULL, 0, NULL, NULL},
   };
 
@@ -10083,7 +10083,7 @@ static void rna_def_geometry_nodetree(BlenderRNA *brna)
   RNA_def_struct_ui_text(
       srna, "Geometry Node Tree", "Node tree consisting of linked nodes used for geometries");
   RNA_def_struct_sdna(srna, "bNodeTree");
-  RNA_def_struct_ui_icon(srna, ICON_MESH_DATA); /* TODO: Use correct icon. */
+  RNA_def_struct_ui_icon(srna, ICON_NODETREE);
 }
 
 static StructRNA *define_specific_node(BlenderRNA *brna,

source/blender/makesrna/intern/rna_space.c

@@ -5658,7 +5658,7 @@ static void rna_def_fileselect_idfilter(BlenderRNA *brna)
       {FILTER_ID_AR | FILTER_ID_CU | FILTER_ID_LT | FILTER_ID_MB | FILTER_ID_ME | FILTER_ID_HA |
            FILTER_ID_PT | FILTER_ID_VO,
        "category_geometry",
-       ICON_MESH_DATA,
+       ICON_NODETREE,
        "Geometry",
        "Show meshes, curves, lattice, armatures and metaballs data"},
       {FILTER_ID_LS | FILTER_ID_MA | FILTER_ID_NT | FILTER_ID_TE,

source/blender/modifiers/intern/MOD_nodes.cc

@@ -1053,7 +1053,7 @@ ModifierTypeInfo modifierType_Nodes = {
     static_cast<ModifierTypeFlag>(eModifierTypeFlag_AcceptsMesh |
                                   eModifierTypeFlag_SupportsEditmode |
                                   eModifierTypeFlag_EnableInEditmode),
-    /* icon */ ICON_MESH_DATA, /* TODO: Use correct icon. */
+    /* icon */ ICON_NODETREE,
 
     /* copyData */ copyData,