Improve labels, description and add container_class

Closes #3470636

config/install/menu_link_attributes.config.yml

 attributes:
+  container_class:
+    label: 'Container class(es)'
+    description: 'CSS class for the menu list item (<li>). Separate multiple classes by space.'
   class:
-    label: ''
-    description: ''
+    label: 'Link class(es)'
+    description: 'CSS class for the link (<a href>). Separate multiple classes by space.'
   target:
-    label: ''
+    label: 'Link target'
     description: ''
     options:
       _blank: 'New window (_blank)'