scale in slippy map

[jstein]

It would be nice to have a scale in the slippy map on download frame.

Due to the projection the scale is not homogenous if the user zooms out to the whole world.
But at scales that allow download it is usefull.

The scale should be calculated for
a) the center of view
or
b) the position of scale.

It should have SI units on the axis or mile and meter in one graph.

||_||| km
| | | miles

​http://en.wikipedia.org/wiki/Scale_%28map%29

[simon04]

In 16894/josm:

fix #4888 - SlippyMapBBoxChooser: show map scale