FIX #527 : Add Thumbnails to 'Choose another file' dialogue

Very difficult to choose an image based on file name alone. Added thumbnail and reduced rows from 10 to 8 to compensate for increased height of rows with thumbnail included.

Will send a separate pull request to the CMS project to deal with styling the gridfield like Asset Admin.

This fixes https://github.com/silverstripe/silverstripe-cms/issues/527