iOS: Add support for delivering touch pressure on iPhone 6s/6s+ devices

As 3D touch can be disabled/enabled at runtime on those devices, we need
to watch for changes to the relevant settings and update the touch device
capabilities that we report to the user.

Note that iOS will deliver touchesBegan with a touch force of 0, which
we will reflect/propagate as a 0 pressure, but there is no clear
alternative, as we don't want to wait for a touchedMoved before
sending a touch press event to Qt, just to have a valid pressure.

Change-Id: I47fb8a9f98ab3244e16a337bbfcf1fe24e4c7aa2
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>