Allow changing the layout of an existing section in Layout Builder UI

(cherry picked from commit bf5c669e)

Closes #3134371

core/modules/layout_builder/css/layout-builder.css

@@ -58,6 +58,10 @@
   background-color: #ffd;
 }
 
+.layout-builder__section .layout-builder__section_links a.layout-builder__link:not(:first-child, :last-child)::after {
+  content: " | ";
+}
+
 .layout-builder__region {
   outline: 2px dashed #2f91da;
 }