Add support for a destructor for ui_out data and use it to

	Add support for a destructor for ui_out data and use it to
	provide a ui_out destructor.
	* ui-out.h: Declare the new ui_out destructor.
	(ui_out_impl): Add a field for data destructor in ui_out_impl.
	* ui-out.c (default_data_destroy): Add a default data destructor
	which does nothing.
	(default_ui_out_impl): Set the new data_destroy field to
	default_data_destroy
	(uo_data_destroy): Local function which invokes the data
	destructor if present.
	(clear_table): Local function which clears the table data of a
	ui_out object.
	(ui_out_destroy): Public function which frees a ui_out object.
	(ui_out_table_end): Use the new clear_table function.
	* cli-out.c (cli_ui_out_impl): Set the new data_destroy field to
	NULL.
	* mi/mi-out.c (mi_ui_out_impl): Set the new data_destroy field
	to NULL.