Refactor transform tracking to support OpenXR

## OpenVR

Transforms for controllers, HMD, and trackers can optionally be created and added to the scene.

The update of the transform is done using the following API:

https://github.com/KitwareMedical/SlicerVirtualReality/blob/53a9549ce19b398d9f0dcca158deaa653ddf91e1/VirtualReality/Widgets/qMRMLVirtualRealityView_p.h#L92-L95

all transform nodes have the following attributes in common:

```
VirtualReality.PoseValid (`True` or `False`)
VirtualReality.PoseStatus (`Uninitialized`, `CalibratingInProgress`, `CalibratingOutOfRange`, `RunningOk`, `RunningOutOfRange`)
```

### Controllers

Based on the state of the controller, the following MRML attributes are set:

```
VirtualReality.ControllerActive
VirtualReality.ControllerConnected
```

Additionally, the position of each controller is updated leveraging the OpenVR specific API `vtkOpenVRRenderWindow::GetOpenVRPose()`

### HMD

Based on the retrieve state, the attribute `VirtualReality.HMDActive` is set:

https://github.com/KitwareMedical/SlicerVirtualReality/blob/53a9549ce19b398d9f0dcca158deaa653ddf91e1/VirtualReality/Widgets/qMRMLVirtualRealityView.cxx#L576-L583

### Generic trackers

In additional of the common attributes, the transform nodes for the generic trackers are named "VirtualReality.GenericTracker" and set with these attributes:

```
VirtualReality.VRDeviceID
VirtualReality.TrackerActive
```

Related issues:
* https://github.com/KitwareMedical/SlicerVirtualReality/issues/171

## OpenXR

Since the `vtkRenderingOpenXR` API (and by extension the OpenXR specification ?) does not provide a way to get the state of the controllers & trackers:
* can the validity of the pose be inferred from the associated transform ?
* or do we need to improve `vtkRenderingOpenXR` API ?

To help answer, here are few observations:
- `VirtualReality.TrackerActive`, `VirtualReality.HMDActive` and `VirtualReality.ControllerActive` are set but not used (in `SlicerVirtualReality` extension)
- `VirtualReality.PoseStatus` is used in `qMRMLVirtualRealityTransformWidget::updateWidgetFromMRML` to exit if there are no change compared to the last status, or update the the icon representing the status of each trackers (including HMD and controllers)
- `VirtualReality.VRDeviceID` is set but not used (in `SlicerVirtualReality` extension)

Optionally, to improve the user experience, it would be nice to still have a way to report the status of each controllers & trackers.

Based on the observations, the critical information we need to have available through OpenXR are the controllers and trackers transforms.

cc: @LucasGandel @lassoan @cpinter @adamrankin

Start with the OpenVR transform API referenced in qMRMLVirtualRealityView_p.h and the HMD handling in qMRMLVirtualRealityView.cxx. Compare those paths with the available vtkRenderingOpenXR API, then inspect qMRMLVirtualRealityTransformWidget::updateWidgetFromMRML. Done means the required controller, HMD, and tracker transforms are available through OpenXR, with any supported status reporting defined.

Written by the indexing model from the issue text.