[josm_download_along] code cleanup/refactoring in order to fix plugin tickets

git-svn-id: http://svn.openstreetmap.org/applications/editors/josm/plugins@29744 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4
[josm_download_along] fix #josm2283 - Download GPX data along a way

git-svn-id: http://svn.openstreetmap.org/applications/editors/josm/plugins@29745 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4