Allow loading label images with the same name from different acquisitions

[jluethi]

We currently don't reload label images that are already loaded. This heuristic is hitting edge-cases though, see this one by @nrepina

When I load a region from round-0 and then load the same region from round-1, I get a message saying e.g. “nuc is already loaded” and it does not load the segmentation image from round-1. It seems this is because “nuc” from round-0 is already loaded and it does not handle layers that have the same name. Ideally a unique name is created, e.g. nuc [1], as in default Napari.

Let's figure out a better way to handle this. May also affect loading channels