Dont delete nodes on undo join ways

[tunp]

When joining ways and then doing undo AddFeatureCommand deletes nodes if joined ways were not on different layers. This then breaks any uploading to OSM.
This pull request simply doesn't do AddFeatureCommand if ways are not on different layers.

[Krakonos]

I don't think I understand the problem correctly, too many negations to wrap my head around. Can you provide an example?

I also think doing operations on features on different layers may be the culprit and we should possibly catch it there, though I think the problem you mentions happens if those features are on the same layer.

[tunp]

Sorry for the unclear description.
You are right. This problem only happens when Features are on same layer. Everything seems to work well when joining ways on different layers.

Here's an example:

1. Create two ways that are connected together by a one node. These ways should be on same layer.
2. Join the ways so it becomes a one way.
3. Do undo.
4. Try to upload to server. It should give a reference error.

[Krakonos]

Interesting. I would have guessed re-adding the node would cause more issues than in undo. But your fix looks reasonable, merging that for 0.19. Thanks!