Class-ify ui_out_level

Class-ify ui_out_level

This patch changes struct ui_out_level to be a real C++ class.  No
behavioral changes.

gdb/ChangeLog:

	* ui-out.c (struct ui_out_level): Replace with ...
	(class ui_out_level): ... this.
	(current_level): Update.
	(push_level): Update.
	(pop_level): Update.
	(verify_field): Update.
	(ui_out_new): Update.