NEW UploadField lists all files, shows path info

It doesn't make a lot of sense to limit the listing
to assets/Uploads/, which is the default set through FileField->folderName.
Showing all files regardless of folder makes them easier
to find, users can still opt-in to filtering by folder through the TreeDropdownField.

I've added a new setDisplayFolderName() method instead for this purpose,
which is separate from File->setFolderName() (the actual upload target).

See https://github.com/silverstripe/silverstripe-framework/pull/2064