Inhand overhaul

Credits to Wjohnston for the dmi editing, I just did the dm changes.

Basically, all the inhands and equipped overlays were moved from the big monolithic dmis to each individual object dmi, appended by _l, _r, _back, _belt, etc. The update_icon code has been changed to account for this. Furthermore, icon_state is now the main choice for inhand names, with item_state (along with the new item_state_icon) persisting as a fallback.

Edit: Removed the WIP tag as it is no longer relevant, although I expect no shortage of inhand bugs are still present (because I didn't test every single inhand)