Fix color map for some labels in the labels_layer

[constantinpape]

Is there an option to fix the color for some of the labels in the labels_layer, @sofroniewn?
Ideally something like this:

fixed_colors = {1: `red`, 2: `green`, 3: `blue`}
viewer.add_labels(labels, fixed_colors=fixed_colors)

I have two different an application where this would come in handy:
using the same colors for the edge and point representation of semantic classes and hiding segments with annotations from the segmentation.

[sofroniewn]

Not yet - this has been noted here, napari/napari#716. I can bump this as a the priority for next week.

[constantinpape]

I can bump this as a the priority for next week.

Ok, that would be great!
I will leave a few more comments in the napari issue later.

[constantinpape]

For having the same colors for points and edges using an image layer with fixed color map is actually more appropriate. I could not quite figure out how to set the color map I need though, see #6.