Rename tui_data_item_window::item_no

tui_data_item_window::item_no is misnamed -- it only can be used for a
register, but it references a "display" number as well.  (Based on
other comments I've seen in the past -- most since deleted -- I think
there were plans at one point to display variables in this window as
well.  However, this was never implemented.)

This patch renames this member to be more correct.

gdb/ChangeLog
2020-07-01  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.c (tui_data_window::show_register_group)
	(tui_data_window::check_register_values): Update.
	* tui/tui-regs.h (struct tui_data_item_window) <regno>: Rename
	from item_no.