Add serialisation / deserialisation of the widget tree

[andydotxyz]

Json looks like a good match for our structure. For those items where we are allowing multiple types in one generic store (like canvas.Objects) we may need custom code like demonstrated here: http://gregtrowbridge.com/golang-json-serialization-with-interfaces/

[andydotxyz]

JSON does not seem to work well because of the way containers have a list of different child widgets - XML can convey this more cleanly.
But this is not a priority.

[andydotxyz]

This is actually in, though requires more work testing...
Defyne uses this to save/load GUI.