Use the label visibility and background color properties

[MattRoyle]

Fixes https://github.com/nion-software/nion-internal/issues/304.
Requires nion-software/nionswift#1883.
Sets the text background to white for readability and makes the interval always show the text so it doesn't have to be selected.

[cmeyer]

I'm not sure how we want to handle this - but 'role' could also embody the zlp_interval._graphic.color = "black" code. That would make thickness and ZLP position the same color, defined by 'role'. We should run that by end users before a final decision, but it's an option.

As another general comment, we eventually want to do everything by 'role' and 'style' and 'id' instead of explicit colors and other properties, similar to CSS.

[cmeyer]

Merged with 632ba29