File copy plugin: add option to write metadata to images directly

When digikam is set up to write metadata to sidecar files only, the file copy plugin now offers an option to write metadata to the image files directly while exporting. The setting is hidden and has no effect in case digikam is set up to write metadata to image files.

Note that this change does not affect the code paths when the images need to be resized, as that already always writes metadata to the image file directly.