Dedicated underwear slots

[Mnemotechnician]

Our fork currently uses markings for underwear. Markings are NOT designed to be altered mid-round, and our current solution gluing the whole thing together is pretty terrible from both the UX standpoint and from the technical point of view. By transitioning underwear markings into clothing entities instead, we can improve the user experience and also improve gameplay in certain areas. However, doing so will require multiple steps:

1. Adding new slots to species mobs: Dedicated underwear slots #738
2. Converting existing underwear markings to clothing pieces (TODO)
3. Obsoleting/removing the "modify undies" system, optionally leaving only its integration into the markings system (is it used anywhere else? if not it can be removed as well)