[josm_measurement] fix #josm5416 - area measurement with several selected closed ways (patch by Preferred)

git-svn-id: http://svn.openstreetmap.org/applications/editors/josm/plugins@29577 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4
[josm_measurement] fix #josm4849 - wrong area, somewhat inaccurate length (patch by Preferred)

git-svn-id: http://svn.openstreetmap.org/applications/editors/josm/plugins@29578 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4
[josm_measurement] fix #josm6872 - Don't mix ways and nodes length computation

git-svn-id: http://svn.openstreetmap.org/applications/editors/josm/plugins@29579 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4