[explore] improve metric(s) and groupby(s) controls (#2921)

* [explore] improve metric(s) and groupby(s) controls

- surface verbose_name, description & expression in controls
- [table viz] surface verbose name in table header

* Fixing tests

* Addressing comments

* Fixing tests (once more)

superset/assets/javascripts/components/InfoTooltipWithTrigger.jsx

@@ -6,17 +6,23 @@ import { slugify } from '../modules/utils';
 const propTypes = {
   label: PropTypes.string.isRequired,
   tooltip: PropTypes.string.isRequired,
+  icon: PropTypes.string,
+  className: PropTypes.string,
+};
+const defaultProps = {
+  icon: 'question-circle-o',
 };
 
-export default function InfoTooltipWithTrigger({ label, tooltip }) {
+export default function InfoTooltipWithTrigger({ label, tooltip, icon, className }) {
   return (
     <OverlayTrigger
       placement="right"
       overlay={<Tooltip id={`${slugify(label)}-tooltip`}>{tooltip}</Tooltip>}
     >
-      <i className="fa fa-question-circle-o" />
+      <i className={`fa fa-${icon} ${className}`} />
     </OverlayTrigger>
   );
 }
 
 InfoTooltipWithTrigger.propTypes = propTypes;
+InfoTooltipWithTrigger.defaultProps = defaultProps;