search: Allow providers to return the complete result object

 This makes the existing createResultObject() match its name better
 and avoids oddities like nested StButtons in app search results.
search: Remove DND code from GridSearchResult

Providers that need drag-and-drop behavior can implement this via the
createResultObject() hook (as the app search provider already does), no
need to duplicate that code in the generic result objects
(ListSearchResult already does not implement DND).