ImgSaver: clean up style
ImgSaver: make saveImg return the writer Metadata
ImgSaver: do not attach metadata to ImgPlus object

This reverts commit 27a944b31dbb48bde22c72063ba4c4fffc7f3df8.

The writer's metadata will very likely not accurately reflect the actual
status of the ImgPlus. And using the same keys that ImgOpener does upon
opening a list of ImgPlus objects kills any chance of accessing the
original metadata once an ImgSaver save operation has taken place.

Furthermore, if multiple write operations happen in a multi-threaded
context, it is a race which of them will finally get to set this
property.

To address these issues, Mark Hiner and I instead decided to have
ImgSaver's saveImg methods return the writer Metadata used. That way,
each thread can do what it likes with the resultant Metadata, avoiding
race conditions.