Add option to open in JOSM to a particular bbox

[batpad]

Allow user to zoom into a section on the map and open that section in JOSM.

Use-case: sometimes a changeset is too big to open the whole thing in JOSM. You have zoned in on a smaller area and just want to open that. This saves clicking through to osm, zooming in, and clicking to edit in Remote Control.

cc @geohacker @samely @ajithranka

[samely]

cc. @Rub21