Coordinate pair for object centroid

[pigsonthewing]

When editing an object, it is possible to use 'CTRL + I' to see its metadata.

For a polygon, say Brentford Library (​254134636), this includes a bounding box, in this example:

Bounding box: -0.3077543, 51.4864532, -0.3073412, 51.4867126

It would be convenient, for copying and pasting (e.g. into other map services, or a when writing a Wikipedia article), if the coordinates of the centre point of that box was also returned, as a single coordinate-pair value, e.g.

Bounding box centre: 51.4865834,-0.3075478

For irregular polygons and ways, where the centre point of that box is not on the way/ inside the polygon, the nearest point to the centre, which meets those conditions, could also usefully be returned.

[simon04]

In 6566/josm:

fix #9494 - Advanced object info: add "Center of bounding box", and for ways "Centroid"