Multiple image channels with flexible overlay functionality

[jmrk84]

This would be extremely useful for our machine learning pipeline. Also, it would generally dramatically widen the possibilties with KNOSSOS when it comes to other applications (e.g. light microscopy with different channels). It is easy to do for the loader, but the GUI and viewer part appear to be a bit harder or more work to me. Nevertheless, we should put this on the agenda.

[Optiligence]

flexible: showing multiple layers at the same time with independent alpha sliders, i guess, which would be as much work for the loader as for the rest. switching between different overlays should be possible right away