[rcanvas] support storage in JSON format
[rcanvas] support storage in JSON format

Allows to create JSON data, which can be used together with JSROOT for
offline display of RCanvas data

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root