Emulated Dpad Buttons

[bjornbytes]

Some improvements to #919.

• Adds new DeviceButton values for virtual dpads:
  • thumbstick/up
  • thumbstick/down
  • thumbstick/left
  • thumbstick/right
  • touchpad/up
  • touchpad/down
  • touchpad/left
  • touchpad/right
  • touchpad/center
• Adds support for XR_EXT_dpad_binding. Since this extension has poor runtime support, emulate it if it's unsupported.
• Maps hand tracking microgestures to touchpad dpad buttons instead of the existing dpad buttons (so that the center thumb tap can be exposed...).
  • Not sure about this. Other options might be:
    • Add dpad/center and keep it mapped to "dpad".
    • Add its own swipe/ namespace with swipe/up, swipe/down, etc.
      • For the central thumb tap: swipe/tap? Or a top-level thumbtap button?
• Rename dpup, dpdown, dpleft, dpright to dpad/left, etc. for consistency.

FWIW I'm still kinda on the fence on exposing these at all, the feature seems a little too high level and can be derived from the axis values pretty easily. On the other hand, OpenXR does provide it, it would reduce the verbosity of the input code in the new error screen slightly, and mapping microgestures onto a directional touchpad kinda makes sense. Wanted to sketch out the idea while I think about it more, and allow others to provide feedback.