Mirror and overlay plot support for variable modifications

[lgatto]

The plotting function I generated is fully functional, but is still only specific to plotSpectra(), not adaptable to plotSpectraMirror() and plotSpectraOverlay().

I am thus asking for your input: are these new plot versions interesting/useful to your average user ? If it is, are there any elements I should remove/add ? Should I do a PR to add a new plotting function that accepts modifications or should the goal be to replace the whole plotting environment ? In the case of plotSpectraOverlay(), where do I put the USI/fragmented sequence ? In plotSpectraMirror() : what happens if there are multiple modifications for one spectrum ? A lot of questions arise and I wonder if it's worth developing this for an average user ?

[jorainer]

Also here a more general comment: would it not be possible to add information on the modifications directly as peaks/spectra variables to the Spectra object? the labels function for plotSpectra() or any of the other functions could then extract and visualize that information.