macOS: Use QCocoaWindow native interface for trackpad touch delivery

Toggling trackpad touch delivery through the string-based
nativeResourceFunctionForIntegration() lookup ("registertouchwindow")
has been superseded by a typed enableTrackpadTouchDelivery() function
on the QCocoaWindow native interface. Use that directly, instead of
resolving an untyped function pointer from the platform plugin.

Pick-to: 6.12
Change-Id: I6da08f0691057f24e03aa9300c7291d26db3b571
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>