feat(explore): Drag and drop UX improvements (#13598)

* Create a metric immediately when saved metric is dropped

* Display borders around control boxes when metric or column is dragged

* Fix imports

* Display ghost button

* Rename placeholder to ghostButton

superset-frontend/src/explore/components/controls/DndColumnSelectControl/DndColumnSelect.tsx

@@ -72,7 +72,6 @@ export const DndColumnSelect = (props: LabelProps) => {
 
   return (
     <DndSelectLabel<string | string[], ColumnMeta[]>
-      values={values}
       onDrop={onDrop}
       canDrop={canDrop}
       valuesRenderer={valuesRenderer}