[0.3.0] regression: loading a new image no longer removes the previous ones

[psobolewskiPhD]

I'm of two minds on this one:

• loading different images on top of each other as layers rarely makes sense in the napari model, so removing makes sense. It will also make it simpler to handle derivative layers like Shapes if one omero image is loaded at a time -- omero also doesn't let you view multiple objects at the same time.
• the user can manage their own layers and choose what they want loaded. Removing by default risks them losing derived/annotation layers without a warning.