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>