Default application palette no longer polished by custom style

The default application palette used to be polished by a custom style. Now only specific class palettes are polished. This is a regression in 5.15.0. I believe it was caused by 0a93db4d82c051164923a10e4382b12de9049b45.

The specific use case is setting a proxy style that can extend Mac and Windows styles to override palette colors, since calling `QApplication::setPalette` doesn't work correctly with those styles.

I can come up with a small sample if this isn't immediately obvious.