Change "paint" to "source" and "mapping" to "layer" in OSC messages

[sofian]

OSC messages still use the old nomenclature, which might be confusing. I would change it to the new way of naming things. However I would also keep offering the "paint/mapping" messages for backwards compatibility.

[aalex]

Yes: Let's keep backward compatibility - until we hit the 1.0 milestone, at least.