Adds a method for fetching files by ParentID
Adds a method for fetching files by ParentID

The CMS front-end (GridField etc) reference files by ID. There are situations where the front-end needs to display a list of files and all we have is the file ID (GalleryField uses filename rather than ID internally). This adds the ability to refresh the GalleryField store with a set of files that have the same ParentID.

open-sausages/silverstripe-assets-gallery,silverstripe/silverstripe-asset-admin,open-sausages/silverstripe-asset-admin,silverstripe/silverstripe-asset-admin,open-sausages/silverstripe-asset-admin,open-sausages/silverstripe-assets-gallery