Fix #111601: VSE strip modifier copy ensure unique name

Previously `SEQ_modifier_list_copy` in append mode does not ensure
unique strip name, which will result in duplicated names in target
modifier list, then `strip_modifier_remove(name="something")` can remove
the wrong one later on. Now fixed using `BLI_uniquename`.

Pull Request: https://projects.blender.org/blender/blender/pulls/111602

source/blender/sequencer/intern/modifier.cc

@@ -1553,6 +1553,8 @@ void SEQ_modifier_list_copy(Sequence *seqn, Sequence *seq)
 
     smdn->next = smdn->prev = nullptr;
     BLI_addtail(&seqn->modifiers, smdn);
+    BLI_uniquename(
+        &seqn->modifiers, smdn, "Strip Modifier", '.', offsetof(SequenceModifierData, name), 64);
   }
 }