Persist positions of nodes in schema explorer

The X & Y positions of each schema node should be saved and restored in the schema explorer. Many users will want to arrange the schema in a specific way and have that arrangement available each time they open the schema.

## Considerations

* A schema refresh should make an attempt to preserve positions of existing types
* Changing the layout algorithm may clear any persisted positions
* Re-running the layout would clear any persisted positions
* Might need an undo action