`XR_EXT_dpad_binding`: support trackpad touch dpad

[Supreeeme]

Currently, based on the spec, it seems that dpad bindings for a trackpad being considered activated are either reliant on the click or force components. However, it would also be useful to be able to use the touch component for a dpad, such as on the Vive wands. This also brings into question how you would specify to use the touch component vs the click, and for this I'd suggest moving the dpad_* components to the end of the click/touch, i.e. ../input/trackpad/touch/dpad_up.

[rpavlik-bot]

An issue (number 2369) has been filed to correspond to this issue in the internal Khronos GitLab (Khronos members only: KHR:openxr/openxr#2369 ), to facilitate working group processes.

This GitHub issue will continue to be the main site of discussion.