Rename hooks from "metadata" to "columns"

function extended_file_field_header($columns) {
  $columns = extended_file_field_columns();
  ...

I'll tackle it from here. :)

Given that we may be able to use the file_metadata_types() function in the upload widget, I'm inclined to put this off until #1962930: Make the file upload widget as flexible as the display formatter: alter hook, configurable table columns lands; just in case we find a new use where the word 'column' no longer makes sense in the hook invocation.

Also, both the metadata_types() and metadata() functions were renamed to columns(), so the patch in #2 results in a duplicate function fatal. Just posting here as a backup to my short memory. :)