Use std::vector for ui_out::levels

Use std::vector for ui_out::levels

Convert the levels field of struct ui_out to be a vector of unique_ptr
to ui_out_level.  This way, the ownership of the ui_out_level objects by
the ui_out instance is clear.

gdb/ChangeLog:

	* ui-out.c (ui_out_level_p): Remove typedef.
	(DEF_VEC_P (ui_out_level_p)): Remove definition.
	(struct ui_out) <levels>: Change type to vector of unique_ptr of
	ui_out_level.
	(current_level): Update.
	(push_level): Update.
	(pop_level): Update, don't manually delete the ui_out_level
	instance.
	(ui_out_new): Update.