Add option to configure which measurement units to use in UI

[Claudius]

This enhancement request was created as a follow up to #6515 by ​http://josm.openstreetmap.de/query?reporter=rickmastfan67 - Here's what he wrote:

Would it be possible to let the user to limit which measurements to cycle through? I would personally like to limit it myself to just Miles and Kilometers since I do all my editing in just the US and Canada. And those are the measurements both countries use. I'd prefer not to have to cycle through the other few that are there for other countries if possible just to switch back between the two.

[Claudius]

The first questions I would have about your request: Where and how would you see tis option to be implemented? Within JOSM Preferences? What kind of representation (checkboxes, table) would you suggest for this option?

[Don-vip]

It was ok to deal with it in the same ticket, but another ticket is fine too :)
Actually I already had in mind a solution, by implementing a context menu with right click on the distance field of the status bar. I just don't know yet what the best wording would be. The menu should mimic the behaviour of the map paint styles menu (each entry would enable/disable the system of measurement for the switch cycle)

[Don-vip]

In 6960/josm:

fix #8859, fix #9893 - Change of System of Measurement (SoM) in map status:

• keep current behaviour (left click) but can be deactivated with statusbar.change-system-of-measurement-on-click
• add all SoMs in contextual menu (right-click) to have a direct access without cycling entirely
• notify user when SoM changes (can be deactivated statusbar.notify.change-system-of-measurement)