Don't derive tui_data_item_window from tui_gen_win_info

There's no deep reason that tui_data_item_window should derive from
tui_gen_win_info -- it currently uses a curses window to render, but
that isn't truly needed, and it adds some hacks to other parts of the
TUI.

This patch changes tui_data_item_window so that it doesn't have a base
class, and updates the register window.  This simplifies the code and
enables a subsequent cleanup.

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

	* tui/tui-regs.c (tui_data_window::display_registers_from)
	(tui_data_window::display_registers_from)
	(tui_data_window::first_data_item_displayed)
	(tui_data_window::delete_data_content_windows): Update.
	(tui_data_window::refresh_window, tui_data_window::no_refresh):
	Remove.
	(tui_data_window::check_register_values): Update.
	(tui_data_item_window::rerender): Add parameters.  Update.
	(tui_data_item_window::refresh_window): Remove.
	* tui/tui-data.h (struct tui_gen_win_info) <no_refresh>: No longer
	virtual.
	* tui/tui-regs.h (struct tui_data_item_window): Don't derive from
	tui_gen_win_info.
	<refresh_window, max_height, min_height>: Remove.
	<rerender>: Add parameters.
	<x, y, visible>: New members.
	(struct tui_data_window) <refresh_window, no_refresh>: Remove.
	<m_item_width>: New member.