omero IDs in napari layer metadata

[will-moore]

See https://github.com/tlambert03/napari-omero/pull/14#discussion_r446664444

When opening an Image from OMERO, it will be useful to store the ID of that Image in the layer metadata, E.g. get_omero_metadata() could include

'metadata': {'omero_id': f'Image:{image.id}',}

This can then be used by other code to e.g. load / save ROIs back to OMERO etc.

Since this becomes a potential API with other code, need to think about what we want to store (and not change it without warning / versioning etc).

Simplest example above. But, we might have opened a HCS Well (as we can do with e.g. iviewer) or have other OMERO IDs we wnat to store. so maybe omero_id is not a good key. Or even omero_object: Image:1 since there might be other objects?

Maybe omero_image:1 and omero_well:2 could work?

cc @joshmoore