Label image disappears when performing clustering

[stefanhahmann]

For newbie users of the clusters plotter / napari, it may feel as a bug, if the label image disappears after drawing the first cluster.

The reason why it gets black is that the opacity is automatically set to zero so that the cluster layer can be seen and not be disturbed by the labels layer.

As this auto-setting is not made explicit to the user, the user may think that the labels layer is "broken" after drawing clusters. This is particularly the case, if the option "hide non-selected clusters" is chosen.

I could imagine 4 potential solutions:

1. Make auto opacity setting of the labels layers an opt-in feature with a check-box
2. Set the opacity only to 0,05 (or similar) instead of zero so that image is still visible
3. Show some warning/info to the user that the opacity of the labels layer has been automatically is to zero (and can be set back)
4. Add a description that this happens to https://github.com/BiAPoL/napari-clusters-plotter/blob/main/README.md

Steps to reproduce: