Model consolidation
Model consolidation

- Renamed LayoutElement to DashboardItem, and made it the root of
everything, since Presentation & LayoutElement had the same core
functionality anyway
- DashboardItem.from_json() still needs to go the hell away

jmptrader/tessera,aalpern/tessera,tessera-metrics/tessera,aalpern/tessera,filippog/tessera,section-io/tessera,jmptrader/tessera,jmptrader/tessera,aalpern/tessera,urbanairship/tessera,jmptrader/tessera,urbanairship/tessera,aalpern/tessera,urbanairship/tessera,tessera-metrics/tessera,urbanairship/tessera,Slach/tessera,filippog/tessera,section-io/tessera,aalpern/tessera,section-io/tessera,section-io/tessera,filippog/tessera,tessera-metrics/tessera,Slach/tessera,tessera-metrics/tessera,Slach/tessera,tessera-metrics/tessera,jmptrader/tessera,urbanairship/tessera,Slach/tessera