Simplify register display

Simplify register display

This patch starts with the observation that the code in
tui_data_window::display_registers_from can all be replaced with a
call to resize.  To make this work propertly, it also changes
tui_display_register to be the "rerender" method on
tui_data_item_window.

The refresh_window method is needed due to the use of nested windows
here.  The ncurses man page makes it sound like this is not very well
supported; and experience bears this out: negelecting the touchwin
call in this path will cause the register window to blank when
switching focus.

gdb/ChangeLog
2019-08-30  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.h (struct tui_data_item_window) <rerender,
	refresh_window>: Declare.
	* tui/tui-regs.c (tui_data_window::display_registers_from): Call
	resize.
	(tui_data_item_window::rerender): Rename from
	tui_display_register.
	(tui_data_item_window::refresh_window): New method.
	* tui/tui-layout.c (tui_gen_win_info::resize): Do nothing on
	no-op.