QPointingDevices for the trackpad and mouse are dynamically instantiated on macOS

This is a consequence of aa4ff7b2e9d4523dbc44afad98a4e3570ec6a997 https://codereview.qt-project.org/#/c/205278/

On Linux (with various platform plugins) we have tried to always enumerate the devices when Qt is initialized; and in the past, we always created a single QTouchDevice representing the trackpad, on startup. (Which was a bit wrong because it's like the "core pointer" idea - an anonymous device, when in fact we can really distinguish the devices, and it's useful to know when there are multiple trackpads.)

I'm not sure yet what exactly this is going to affect... but it's now a minor platform difference, and it would be much better if we could find macOS API to enumerate the devices at Qt initialization time, and also be notified when devices are hotplugged, rather than waiting until we get events from them.