Allow for different color plots for each source when using the spectral viewer to view spectra from multiple images

[Adam]

Example: When plotting spectra from two linked images spectra are plotted using the same color and it is difficult to tell which spectra belong to which image.

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[Andreas Janz]

Yes, we will improve this, also see #422

[Andreas Janz]

This will be possible in the next version. Currently there is still an issue with that, see #714.

[Benjamin Jakimow]

now possible with new SpectralLibraryWidget (based on color expression)