Support if-unused attribute in OsmChange.

[blackboxlogic]

I see there is a test to deserialize a a change file where the Delete element has an if-unused attribute, but

• After deserialization, there is no record that the delete was 'if-unused'
• There is no way to generate XML that has 'if-unused'

A description of the feature's purpose can be found here

[blackboxlogic]

There is no clear path for fixing this. If I got guidance, I can make a pull request. The problem is that Deletes are stored as an OsmGeo[] in OsmSharp.Changesets.OsmChange.Delete so there is no room for any more data on the Delete object. It could live on the OsmChange object though. Thoughts?

[xivk]

I'm assigning this to myself and have a look.

An idea: We could introduce an extra object specifically for delete operations that has an implicit cast from OsmGeo for the default cases.