Abstract work

[skinkie]

Based on the discussion in #918 this implements the following (per commit):

1. cdeb8c6 and 4913f4a cause Dummy types to be abstract, hence cannot be used to create (or parse) elements.
2. abstract="false" is the default, removed everything that is the default, cleans up the schema
3. Call_ dummy type was not required. There is no DatedCall that already worked with it.

Yes, this changes a lot of cose, but then again, our aim is not have any changes in the repo anymore.