Minor There are some known limitations with iron-overlay-behavior and the stacking context causing the dialog to appear behind the backdrop.
https://github.com/PolymerElements/paper-dialog/issues/109
https://www.webcomponents.org/element/PolymerElements/iron-overlay-behavior
"You should place this element as a child of <body> whenever possible."
Add a property to nuxeo-dialog to allow it to be moved to <body> when opened (and moved back to the original DOM node when closed), so we can deal with this limitation in some use cases where we want to have a dialog declared inside an element (ex: UI actions elements) but want to open it with <body> as parent.
