Layer visibility in coordinate systems

[melonora]

With #99 the following behaviour is implemented:

• Layers are automatically made invisible when switching to a coordinate system they do not belong in
• If a layer is visible in one or multiple coordinate systems. This information is kept and upon switching back to the coordinate system
  in which a layer was visible it becomes visible again.

The behaviour in point can be overwritten as in a person can click on the eye icon and the layer will become visible even though it does not belong in the coordinate system. This allows for the use case of landmark registration. @giovp @LucaMarconato and I had a discussion regarding what the behaviour should be. Two alternatives have been proposed:

1. We trust the user to know which layer belongs to which coordinate system. Napari-spatialdata would not give information regarding this except with the elements that can be selected in the element widget when having set a particular coordinate system
2. Upon switching to a particular coordinate system, all layers will be visible, even when previously invisible in the particular coordinate system.