Gets the default root panel. This panel wraps body of the browser's
document. This root panel can contain any number of widgets, which will be
laid out in their natural HTML ordering. Many applications, however, will
add a single panel to the RootPanel to provide more structure.
the default RootPanel
Gets the root panel associated with a given browser element. For this to
work, the HTML document into which the application is loaded must have
specified an element with the given id.
Parameters: id - the id of the element to be wrapped with a root panel the root panel, or null if no such element was found