Have an 'upload prohibited' flag in .osm files

[Ldp]

While working with a few layers in JOSM, it's very easy to start an upload from the wrong layer. If you don't catch it right away, it would mean time lost to revert it, a spot of agony, and no matter the other destructive things that could occur to live OSM data.

I propose to add an upload prohibit flag to .osm files, so when JOSM loads one of them in a layer, and you try to upload, it refuses to do so.

For example: <osm version='0.6' upload='no'>

This prohibit status should also be settable on a layer, through the GUI, so one can protect oneself from inadvertently doing stupid things, and should then be written out when saving the layer.

This would also apply neatly to other test files around, like the osmarender test file.