GUI: Allow settings to have a "virtual value"

[pfebrer]

A virtual value would be the equivalent value of the setting to get the actual state of the plot.

This is best understood with an example:

BandsPlot has two settings that choose which bands to display: Erange and bands_range. If none of them are provided, the bands to display are inferred from the fermi level. However, to help the user update these settings (give them a notion of what are the ranges), we set Erange and bands_range to the corresponding values. It would be great that these two settings are not actually set, but just contain a virtual value that is displayed to the user.

Otherwise, one needs to set Erange back to None in order for an update in bands_range to take effect.