add prop for updating blocks from external events

[hcientist]

Similar to the use case in Praxly, I would like to have an external event (like the update of code in another format) result in the blockly workspace re-rendering its state. I propose adding props:

1. updateXML
2. updateJSON

If the value of either of these changes, BlocklyWorkSpace would ideally do much (but perhaps not all?) of what it does in the respective importFrom functions (i.e. importFromXml, importFromJson)