Add a "maxTouchPoints" property to the device object

[tigleym]

Being able to check the device's actual max touch points value via navigator.maxTouchPoints would be nice to have.

In Firefox DevTools' RDM tool, it's very difficult (if not impossible) to know what the simulated touch device's actual max touchpoints value is. While we could fake the value to be 1 for all touch devices, it would provide an inaccurate experience. For now, this issue could assume that all touches have max touchpoints of at least one.

See issue at: https://bugzilla.mozilla.org/show_bug.cgi?id=1500672