Register mouse devices on cocoa

It would have been nice to ensure that a device is registered already in
mouseEnteredImpl(); but in that context, NSEvent.deviceID is always 0,
and we can't find out anything else about the device.
QWindowSystemInterface::handleEnterEvent() doesn't currently take a
QPointingDevice either.

In handleMouseEvent() and scrollWheel(), deviceID seems unique for each
trackpad or Magic Mouse, but 0 for any plain USB mouse. There, the first
mouse that the user interacts with becomes primaryPointingDevice():
its deviceID is assigned to systemID (except if deviceID == 0, we use
1 instead, to avoid the auto-incrementing device ID assignment in the
QInputDevicePrivate ctor.) When scrolling occurs, we update the
capabilities to have PixelScroll if theEvent.hasPreciseScrollingDeltas.

So over time, QInputDevice::devices() should build up to a complete
list, with capabilities() also distinguishing plain mice from those that
have the PixelScroll capability. And in the common case that the user
has only one Apple pointing device, it becomes primaryPointingDevice().

Pick-to: 6.2
Task-number: QTBUG-46412
Task-number: QTBUG-63363
Task-number: QTBUG-72167
Change-Id: Id9771b4dfd765e49023bd57d42a2aa4d0635a3b2
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

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