required contentblocks

• check out the demo (https://forge.onehippo.org/svn/content-blocks/demo/tags/content-blocks-demo-1.11.01)
• open http://localhost:8080/cms/console/?path=/hippo:namespaces/contentblocksdemo/contentblocksdocument/hipposysedit:nodetype/hipposysedit:nodetype/contentblocks
  and add the property "hipposysedit:mandatory" and set it to true
• go to http://localhost:8080/cms/?1&path=/hippo:namespaces/contentblocksdemo/contentblocksdocument
• delete all content blocks and save the document.

Current situation:
No validation message is shown

Desired situation:
A validation message is shown.