Issue #3406382: Support "Always visible" columns

Closes #3406382

config/schema/flexible_views.style.schema.yml

@@ -18,9 +18,9 @@ views.style.flexible_table:
         type: mapping
         label:  'Column info'
         mapping:
-          default_visible:
-            type: boolean
-            label: 'Column visible on default'
+          visibility:
+            type: string
+            label: 'Column visibility'
           sortable:
             type: boolean
             label: 'Sortable'