Update OpenBrush to use Unity XR plugin system

[billyquith]

Unity 2020.x deprecates its VR support in favour of a new XR plug-in framework. OpenBrush is currently on Unity 2019.4 LTS. In order that we can keep up with the latest features in Unity, and stop the code from going stale we will need to update the VR SDK, so that we can later move to Unity 2020, 2021, etc. This issue just addresses the XR plug-in work.

This work is being done in the feature/xr_plugin branch. It is hoped this work will be completed with 2 months.

Notes about moving to Unity XR plugins. There are some complications here. See #130.

• We can move to XR plugin framework to support some vendors and we can stick on SteamVR plugin to support others.
  • We can stick on Unity 2019.4 LTS for this.
• When we move to Unity 2020.3 we can use the Unity OpenXR XR plugin.

[billyquith]

Related to #222