SelectApp: factor out Vis from Schema

[gwk]

Currently, pithy.sqlite.schema provides a Vis class, and Column takes a vis argument. This is used to provide customized rendering of table cells. Since Vis is only relevant to SelectApp, we should move it to pithy.web.asgi.select, and remove the Column.vis property. The main reason to do this is because because it will reduce import tangles; schema definitions should not be dependent on the visualization code.

Instead, SelectApp should take a constructor argument vis that is a nested dictionary mapping schema -> table -> column -> Vis instances.

[davlis-tr]

PR #10

[gwk]

This code was successfully merged, just not with GitHub.