Add class-based rows selected method

[gshotwell]

We want to have a method for getting the selected rows from a data frame. This will help with discoverability over the current implementation.

Current

ui.output_data_frame("my_df")
input.my_df_selected_rows()

Future

my_df.input_selected_rows()

Possible future methods

my_df.update_selected_rows
my_df.input_data_selected()
my_df.update_cell()
my_df.update_row()
my_df.input_cell()