Make the XWiki.Document JS API usable before the xwiki:dom:loading event

The XWiki.Document API relies on class properties that are initialized during the xwiki:dom:loading phase, which makes it unusable from other code that gets executed before that event, or even from code that hooks into the same event if the event listener order isn't preserved. Since the DOM elements used by this widget are already loaded (meta elements placed before scripts in the head), there's no need to delay the initialization.