add filelink sorting direction field to gallery content element

# Feature Request

In the gallery content element you already have a field for various sorting options (filename, filesize, last modification date, etc).
An editor asked if it would be possible to sort the image in an descending (filename) order?

There is no sorting direction field in this element.

You can add `sorting.direction = descending` to the fileprocessor but then every gallery is sorted in that direction.

It would be better to have a dedicated field, where you can change the sorting direction, like you have in the uploads field with `sorting.direction.field = filelink_sorting_direction`

Would it be possible to add that same field to the gallery content element?

```
tt_content.gallery {

################
### TEMPLATE ###
################
templateName = Gallery

##########################
### DATA PREPROCESSING ###
##########################
dataProcessing {
10 = TYPO3\CMS\Frontend\DataProcessing\FilesProcessor
10 {
references.fieldName = image
folders.field = file_folder
sorting.field = filelink_sorting
sorting.direction = descending
# sorting.direction.field = filelink_sorting_direction
}
}
}
```

Start by locating the gallery content-element configuration and its FilesProcessor sorting setup described in the issue. Compare it with the uploads field's existing sorting-direction field. Done means editors can select a sorting direction per gallery without changing the global default.

Written by the indexing model from the issue text.