Napari: Make color mode for label layers more informative

[Czaki]

🐛 Bug

Current implementation of color mode does not provide user enough information what he should expect after change this option.
It allow to select direct when user does not provide mapping dict.
With current implementation also order of changing properties produce different result in viewer which will introduce problem with #1584 and simple screenshot of layer control widget is not enough to reproduce view settings.

To Reproduce

Steps to reproduce the behavior:

1. select auto
2. select selected
3. select direct
4. select selected
   

Expected behavior

Rendering labels layer should not depend on history

Environment

napari 0.3.7rc1

• Please copy and paste the information at napari info option in help menubar here:

• Any other relevant information:

Additional context

First information on zulip
https://napari.zulipchat.com/#narrow/stream/215289-release/topic/0.2E3.2E7/near/208787455

My suggestion is to split this widget on two. One for control selection and one for colormap.
Or make 4 positions in this widget. 4-th one should be select with direct

[sofroniewn]

This is being addressed at least in part by #1762 I think, right @DragaDoncila?

[DragaDoncila]

That's right, at least the most confusing part - the selected mode, will be removed in favour of the checkbox

[sofroniewn]

This has been fixed by #1762 I think @Czaki and so I will close, but feel free to reopen if not fully addressed or make a new issue for the remaining parts. Thanks!!