validate function is called twice in create-layout

Steps to reproduce:

• Have a server running (Nuxeo OOTB is fine)
• Connect to it; navigate to a container where you can create a File (Workspace or Folder, typically)
• Open nxserver/nuxeo.war/ui/document/nuxeo-file-create-layout.html
• In the script, add this validate function, after the properties object:

    . . .
      properties: {
        document: Object
      },
      validate: function() {
        console.log("Validate");
        return true;
      }
    . . .

• Shift-Reload the page
• Display the creation dialog
• Show the WebInspector, in the Console tab
• Create a new File document, setup the title, click the "Create" button
• See "validate" is displayed twice.

Environment  is Nuxeo 10.10 with HF04

Workaround involves a flag, it's not elegant , and more importantly, maybe it reveals some performance/logic issues:

      properties: {
        document: Object,
        validateDoneOnce: {
          type: Boolean,
          value: false
        }
      },
      validate: function() {
        if(!this.validateDoneOnce) {
          console.log("Validate");
          this.validateDoneOnce = true;
        }
        return true;
      }