Add table support back into the Webform UI

Problem/Motivation

Besides multiple composite elements, advanced users should be able to build custom table containing individual elements.

@see https://drupal.stackexchange.com/questions/258478/populating-webform-tables

Proposed resolution

Add table support back into the Webform UI

Here a prototype of the YAML source

table:
 '#type': table
 '#header':
   - title: 'First Name'
     attributes: { }
   - title: 'Last Name'
     attributes: { }
   - title: Gender
     attributes: { }
   - title: 'US Citizen'
     attributes: { }
 '#row__key': table_row_{00}
 '#row__title': Table row {0}
 '#row__elements':
   table_{00}_first_name:
     '#title': '{0} - First name'
     '#title_display': invisible
     '#type': textfield
     '#size': 30
   table_{00}_last_name:
     '#title': '{0} - Last name'
     '#title_display': invisible
     '#type': textfield
     '#size': 30

Remaining tasks

• Add markup support to composite widget
• Add table webform element
• Allow row elements to created in UI (w/ markup)
• You must use 00 in the name of the row element so that each row element is incremented correctly.
• Only allows table row elements to be added as children to table.
• Add YAML source validation which ensures that 'table_row' can only be added to a 'table'
• When table row is added to a table copy the elements and add indexes.
• \Drupal\webform\Plugin\WebformElementInterface::getConfigurationFormProperties
• Write tests
• Write change record
• Create video

User interface changes

• webform_table element
• webform_table_row element
• 'Add row' page
• Update the composite element builder

API changes

N/A

Data model changes

N/A

Release notes snippet

TBD

User Acceptance Test

• Confirm Table row is not visible in 'Add element' UX.
  /admin/structure/webform/manage/table_test/element/add
• Create a test webform
  /admin/structure/webform
• Add Table element
• Add Table row element
• Add Textfield to table row
• Add Elements to table row
• Add Table row element with duplicate checked
• Enable for Preview
  /admin/structure/webform/manage/{webform}/settings/form
• Test the form and click 'Preview'
  /webform/{webform}/test
• Check Table element #states support
• Check Table row element #states support