[BUG]String and caption are reversed in message boxes in macOS

This fixes it:
```cpp
EMsgBoxResult _ShowMessageBox(iplug::igraphics::IGraphics* pGraphics, const char* str, const char* caption, EMsgBoxType type) {
#ifdef OS_MAC
  // Apple is backwards?
  return pGraphics->ShowMessageBox(caption, str, type);
#elif defined OS_WIN
  return pGraphics->ShowMessageBox(str, caption, type);
#else
#error NOT IMPLEMENTED
#endif
}
```