Support System Button in Knuckles Bindings

[prikhi]

Modify the profiles::knuckles module to allow binding of actions to system/touch & system/click for both hands. No transformation is necessary as the path mapping is identical between the OpenXR spec & SteamVR.

Note that OpenXR runtimes may choose to ignore bindings to the system buttons.

[prikhi]

click & touch for both hands as per openxr spec: https://registry.khronos.org/OpenXR/specs/1.1/html/xrspec.html#_valve_index_controller_profile

and as in steamvr drivers:

// driver/.../index_controller_profile.json
  "input_source" :
  {
    "/input/system" : {
      "type" : "button",
      "click" : true,
      "touch" : true,
      "binding_image_point" : [ 34,45 ],
      "order" : 1
    },
// ...

With Moss 2 binding file as an example:

            {
               "inputs" : {
                  "click" : {
                     "output" : "/actions/main/in/ui_menu"
                  }
               },
               "mode" : "button",
               "path" : "/user/hand/left/input/system"
            },

[prikhi]

ugh... I can add back the tabs if desired...
(forgot to turn nvim autoformat of json off)

[Supreeeme]

The spec says that system buttons may not be available for application use. In practice what this means is that depending on what runtime it's running under, suggesting bindings using the system button may actually error out with XR_ERROR_PATH_UNSUPPORTED. For this reason, along with the fact that I'm pretty sure even binding to the system button in SteamVR requires changing some setting, I opted to not use system button bindings. I'm not really against supporting them at all, but this case needs to be handled.

[prikhi]

In practice [...] may actually error out with XR_ERROR_PATH_UNSUPPORTED

Ah good to know, the spec says it "must" accept it but is free to ignore it so figured it wouldn't cause any problems:
https://registry.khronos.org/OpenXR/specs/1.1/man/html/xrSuggestInteractionProfileBindings.html

I'm not really against supporting them at all, but this case needs to be handled.

aight thanks, will poke around to see where we're doing this & if the error can be handled cleanly.