public JQMButton setLeftButton(String text, String url) {
        return setLeftButton(text, url, null);
    }
public JQMButton setLeftButton ( String text , String url ) { return setLeftButton ( text , url , null ) ; }
Creates a new {@link JQMButton} with the given text and linking to the
given url and then sets that button in the left slot. Any existing
right button will be replaced.

@param text
the text for the button
@param url
the optional url for the button, if null then this button
does not navigate by default

@return the created button