Class-ify ui_out_hdr

Class-ify ui_out_hdr

This patch makes ui_out_hdr (the object that represents an ui-out table
header) a proper C++ class.  No behavior changes, it's all about
encapsulation.

gdb/ChangeLog:

	* ui-out.c (struct ui_out_hdr): Replace with ...
	(class ui_out_hdr): ... this.
	(append_header_to_list): Update.
	(get_next_header): Update.
	(ui_out_query_field): Update.