Add non-action function parsing to the DECORATE parser
Add FxVMFunctionCall class

- This replaces the general extensibility that had existed formerly
  in thingdef_function.cpp. Parameter parsing for function calls is
  shared with state parameter parsing. Functions are defined exactly in
  the same way as action functions, but without the 'action' keyword.
Reimplement CheckClass and IsPointerEqual functions

- The definition of FxGlobalFunctionCall_CheckClass was big and scary. I
  would say this is a big improvement, since now it gets to leverage the
  same framework that action functions use.
- The definition of FxGlobalFunctionCall_IsPointerEqual was not so big and
  scary, so this can't be called so much of an improvement as
  CheckClass was. (Which is not to say that it isn't better anyway.)