Selected rows method

[gshotwell]

Closes #1120

[schloerke]

• Get make pyright to pass

[wch]

A couple module-related questions:

• Does this work in a module?
• Does it work if there's a module which returns the renderer object? For example, suppose the module is something like this, and then the server function that instantiates the module wants to get the selected rows:

  @module.server
  def mod_server(input: Inputs, output: Outputs, session: Session): 
      @render.data_frame
      def grid():
          ...
  
      return grid.selected_rows