UI: Show material link type icon in dropdown

Enums that are displayed as icon-only should have their icons defined
in RNA, not just through the layout in Python. That way they are shown
in the dropdown itself and users make the connection between the listed
item label and the icon while collapsed.

scripts/startup/bl_ui/properties_material.py

@@ -130,8 +130,7 @@ class EEVEE_MATERIAL_PT_context_material(MaterialButtonsPanel, Panel):
             row.template_ID(ob, "active_material", new="material.new")
 
             if slot:
-                icon_link = 'MESH_DATA' if slot.link == 'DATA' else 'OBJECT_DATA'
-                row.prop(slot, "link", icon=icon_link, icon_only=True)
+                row.prop(slot, "link", icon_only=True)
 
             if ob.mode == 'EDIT':
                 row = layout.row(align=True)

source/blender/makesrna/intern/rna_object.cc

@@ -2337,8 +2337,8 @@ static void rna_def_material_slot(BlenderRNA *brna)
   PropertyRNA *prop;
 
   static const EnumPropertyItem link_items[] = {
-      {1, "OBJECT", 0, "Object", ""},
-      {0, "DATA", 0, "Data", ""},
+      {1, "OBJECT", ICON_OBJECT_DATAMODE, "Object", ""},
+      {0, "DATA", ICON_MESH_DATA, "Data", ""},
       {0, nullptr, 0, nullptr, nullptr},
   };