Fix increasing load time of listing when adding multiple 'bulk edit' buttons

When adding bulk edit buttons to WebUI using guide HOWTO: Create a Bulk Edit Form Using Nuxeo Studio - Create a Bulk Edit Layout the loading time of the page increases dramatically for each button added.

How to reproduce:

• in Studio Modeler, define the Contract document type as explained in HOWTO: Customize Document Layouts - Create a Contract Document Type
• in Studio Designer, define its corresponding layouts, especially the edit layout
• define 6 to 10 bulk edit buttons as explained in HOWTO: Create a Bulk Edit Form Using Nuxeo Studio - Create a Bulk Edit Layout
• fire up a 2021 instance with addon nuxeo-web-ui-3.0.11 and the Studio customizations
• Add 400 documents to a workspace:

  DOCTYPE=File
  for ((i=0; i<400; i++))
  do
      printf -v idx "%09d" $i
      docname=${DOCTYPE}_${idx}
      curl -XPOST -su Administrator:Administrator \
  -H'content-type: application/json' -H'properties:*' -H'Repository:default' \
  http://localhost:8080/nuxeo/api/v1/path/default-domain/workspaces/ws1 \
  -d '{
    "entity-type": "document",
    "type": "'${DOCTYPE}'",
    "name": "'${docname}'",
    "properties": {
      "dc:title": "'${docname}'"
    }
  }' \
  | jq -r '.path // .stacktrace'
  done
  

• navigate to the workspace ws1

Expected result: listing of the 400 documents with 6 to 10 bulk edit buttons takes as much time as if there was only 1
Actual result: load time increases (probably geometrically) for each bulk edit button added