Anim: fix bone collection custom property copying on armature join

The armature join operation should preserve custom properties,
as described in the comment in the code. Previously this was
achieved by reusing the collection instances from the armature
being joined, but it was changed to create a new instance. However,
property copying was not implemented, which breaks Rigify.

This fix adds property copying behavior back.

Pull Request: https://projects.blender.org/blender/blender/pulls/116626

source/blender/editors/armature/armature_relations.cc

@@ -30,6 +30,7 @@
 #include "BKE_constraint.h"
 #include "BKE_context.hh"
 #include "BKE_fcurve_driver.h"
+#include "BKE_idprop.h"
 #include "BKE_layer.h"
 #include "BKE_main.hh"
 #include "BKE_report.h"
@@ -339,6 +340,9 @@ int ED_armature_join_objects_exec(bContext *C, wmOperator *op)
         BoneCollection *mapped = bone_collection_by_name.lookup_default(bcoll->name, nullptr);
         if (!mapped) {
           BoneCollection *new_bcoll = ANIM_armature_bonecoll_new(arm, bcoll->name);
+          if (bcoll->prop) {
+            new_bcoll->prop = IDP_CopyProperty_ex(bcoll->prop, 0);
+          }
           bone_collection_by_name.add(bcoll->name, new_bcoll);
           mapped = new_bcoll;
         }