Add two methods to tui_data_window

Add two methods to tui_data_window

This changes tui_show_registers and tui_show_register_group to be
methods on tui_data_window.

2019-08-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.h (struct tui_data_window) <show_registers,
	show_register_group>: Declare.
	(tui_show_register_group): Don't declare.
	* tui/tui-regs.c (tui_data_window::show_registers): Rename from
	tui_show_registers.
	(tui_data_window::show_register_group): Rename from
	tui_show_register_group.
	(tui_data_window::check_register_values, tui_reg_command):
	Update.
	* tui/tui-layout.c (tui_set_layout): Update.