rename GetClassName/IsClass, declare mxtypes

Was intended as a simple code improvement, however it also seems to make WinMain, MxString::operator=, MxDSFile::Open 100% (all of which just needed registers to be switched around)

LEGO1/legoomni.cpp

@@ -99,7 +99,7 @@ void LegoOmni::vtable24(MxDSAction &ds)
 MxBool LegoOmni::vtable28(MxDSAction &ds)
 {
   // FIXME: Stub
-  return MX_TRUE;
+  return TRUE;
 }
 
 void LegoOmni::vtable2c()