(Backport) Issue #6753: WebP always exports Thumbnail Exif metadata

Attempts to port 2.99's solution to issue #6753 (closed) to 2.10, by allowing users to toggle saving thumbnail Exif metadata, as shown here:

A new attribute was added to WebPSaveParams for thumbnail metadata and then connected to a GUI checkbox. An additional Script-fu procedure was also created to allow non-interactive control of thumbnail metadata saving as well.