rcxml: use fixed arrays for rc.title_buttons_*

rcxml: use fixed arrays for rc.title_buttons_*

These are just lists of enum lab_node_type, with a bounded size and
no middle-insertions/removals, so linked lists are overkill.

Also, the use of wl_list_for_each[_reverse] just to access the first or
last entry in the list (corner button) was weird.