[w3c/gamepad] Support trackpad inputs (#34)

Many controllers now include trackpads.  Some examples:

- PS4 controllers include a rectangular, capacitive touch, clickable trackpad in the center of the gamepad
- HTC Vive controllers include a round, capacitive touch, clickable trackpad in the thumb position.
- The "Steam Controller" includes two separate, rount, capacitive touch, clickable trackpads.

I would like to extend the Gamepad interface to enumerate the number of trackpads, their shape, arrangement, and capabilities.  Capabilities would include the ability to click and ability to detect multiple touches.

This issue is driven initially by the need to support the HTC Vive Controllers for WebVR.  The trackpad is fundamental to the usage of the controllers and necessary for the established VR UX paradigms for the platform.

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