[Request] Expose new stuff

[KernCore91]

This is a request to expose:
studio.h,
com_model.h,
model_t,
studiohdr_t and
studio
That would allow easy model access.
Also gamerules, with a easy option to add our own rules.
Access to Player Model Overrides if it's not there, along with UserInfo Changed #12 and EntityCreated #10 hooks.