Display straight distance while drawing

[bagage]

A feature request a bit related to https://github.com/nrenner/brouter-web/issues/68 (but this one is much smaller):

When drawing a line, it would be nice to display the distance from the previous point, to get a quick estimation of the distance. Something (very) roughly like this:

[nrenner]

I personally fear this might get in the way or be distracting.

So ideally there would be a way to turn this off (e.g. as a general option or as a mode button in a secondary toolbar/navigation that is only shown while drawing, as drafted in the 68-sl-routing branch).