AbstractImageMetadata: use javadoc syntax
ImageMetadata: implement the Named interface

Each associated Image should be able to assign a name.

This will be really useful for formats like the OMEROFormat, which gets
the name from the database rather than from the source string.
ImgSaver: do not overwrite the name or source

Just because we wrote the image out to a particular destination does not
give permission to overwrite metadata from the original source image.
ImgOpener: set name of ImgPlus from ImageMetadata