Idea: Add .sql and .data() methods to MosaicWidget

[manzt]

It would be useful to have a convenient API to materialize the SQL for a current vgplot in Python. For example:

widget = MosaicWidget(spec, data={"weather": df})
widget # interact with the widget, (i.e., cross filter some points)

widget.sql # access the current sql string
widget.data() # materialize the sql in Python

I've implemented something like this for a custom data table client I've been working on, and was looking to maybe upstream a PR to mosaic up following a conversation with @domoritz today.

However, it's not totally clear what .sql would point to for a given spec. In the case where this is just one data source, you could apply all the selections and params as filters to that source. Or something named like:

widget.data("weather") # applies active selections/params

If there are multiple sources.