widget: fixed "no-boundingbox-at-start" issue
widget: fixed "no-boundingbox-at-start" issue

Instead of the application to watch out for a valid bbox, the widget does
it itself.

o added new callback onDocumentReady(), that may be overriden. Called
  when the target's bbox is not empty
o widget's defs elements are now updated when document's ready

xml3d/xml3d.tools.js