Rename tui_data_item_window -> tui_register_info

tui_data_item_window used to hold a curses window, but we removed that
ages ago.  Now it just holds information about a single register.
This patch renames the class to make it more clearly reflect its
meaning.

Tested-By: Tom de Vries <tdevries@suse.de>
Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>