Useful scalebar values

[normanrz]

Detailed Description

Make the scale bar display useful sizes. This implies that the scale bar will get a variable width depending on the exact zoom level. However, it should have a max width. For inspiration: https://github.com/google/neuroglancer/blob/master/src/neuroglancer/widget/scale_bar.ts

Context

The scale bar has a fixed width which results in fractional values. That makes it hard to use the scale bar for roughly measuring objects in the data.