Store sourceId for widgets

Since we are supporting dataviews having an analysis as a type of source (https://github.com/CartoDB/cartodb.js/issues/1212), we now need to store the sourceId (the id of the analysis) whenever we create a widget.

/cc @alonsogarciapablo

[juanignaciosl]

@javierarce frontend currently sends source_id: id (current status of branch at #7217). It’s not a problem for me, but maybe you’d like to send source: { id: id } instead so you handle it the same way for requests and responses. That will be the format used at vizjson and named map template as well.

[juanignaciosl]

After talking to @rochoa he confirmed that this should send this to Maps API:

{
  options: { ... },
  source: { 'id': id },
  type
}