Block::sources(): use sourceCount() and getSources()
Block: implement findSources()
EntityWithSources::sources use Block::findSources()
EntityWithSources: Implement new source_ids()
EntityWithSources::hasSource: Use source_ids()
EntityWithSources::sources(): use source_ids()
TestDataArray: include basic sources() test
Block: remove existsSource()
Block: remove findSource()
DataArray: remove setData()