Use CoreUI to draw the title bar and separator if available.

This uses private APIs and so is disabled by default, but for developers
who are not releasing their apps in the App Store (or have a separate
non App Store build), INAPPSTOREWINDOW_USE_COREUI can be set to 1 to
draw 100% native title bars with graceful fallback to custom drawing if
the CoreUI drawing function could not be loaded.