Visualize unassociated channels better

[t-b]

When running unassociated DA/AD channels, it can be difficult to remember which are unassoc and which are associated. @MichaelHuth had the idea to colorcode the AD/DA channels on the DA/AD tabs to indicate that.

We could reuse the headstage colors, including green for unassoc, for that.

[timjarsky]

Instead of color, label the channels with the corresponding headstage.