`XR_EXT_dpad_binding`: support trackpad touch dpad

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`.

Start with the XR_EXT_dpad_binding specification and examine how trackpad click, touch, force, and dpad components are currently represented. Done means the specification has a decided, documented way to activate a dpad from trackpad touch and to distinguish touch-based paths from click-based paths.

Written by the indexing model from the issue text.