Setting data and color_indices (via code or GUI) should update all artists data and colors

[zoccoler]

Consider adding a selected_indices signal to Interactive class that can be connected to each artist color_indices.

[zoccoler]

Basically, data and color_indices properties from both artist need to be linked somehow, probably via pysignal.

[zoccoler]

Not sure anymore if this should be done here or externally.