Open up player skin setting to ZScript and/or ACS

[violgamba]

I play coop a lot, frequently making my own coop-centric maps. We have a mod with a collection of skins to choose from and it'd be fun if, rather than navigating the menu system, a map could include a line mirror and a switch to progress the player's skin.

Unfortunately, while ZScript wraps the engine's GetSkin() function, it does not wrap the engine's SkinNumChanged(int skinnum) function.

I THINK it'd be a simple thing to add a ZScript wrapper for SkinNumChanged(int skinnum) and would allow for a fun feature for multiplayer maps. I'd even be willing to implement this and make a pull request if the dev's give their consent.

[violgamba]

I've submitted a pull request for this here:
viewtopic.php?t=79169
https://github.com/ZDoom/gzdoom/pull/2546