Add number to changeset comment for large uploads spanning multiple changests

[Mashin]

When uploading large amount of data (typically imports) that has to be uploaded in multiple changesets it would be a neat feature if JOSM would number the changesets.

So the changeset commets would have the typed in comment + automatically added number at the end and look like e.g.:
<user defined comment> 1/12
<user defined comment> 2/12
<user defined comment> 3/12
...
<user defined comment> 12/12