…XR enabled

Standalone mode is primarily intended for SteamVR overlays where it is undesirable for Unity to initialize itself as a VR application and render a scene.

In order to run standalone:

- autoEnableVR must be disabled in SteamVR_Settings
- "Virtual Reality Supported" must be disabled in Unity's project settings
- openvr_api.dll will not be handled by Unity so the dlls for the relevant platforms must be downloaded and included in the project with the proper import settings (you may need to manually copy the dlls into the standalone build version)
- To initialize an overlay you must create a behaviour that calls `SteamVR.InitializeStandalone(EVRApplicationType.VRApplication_Overlay);` to enable SteamVR functionality
- If you use SteamVR scripts that initialize SteamVR on Awake you may get initialization warnings if they run before your initialize behaviour
- Since Unity's XR support is disabled you of course need to interact with the `OpenVR.overlay` directly in your own scripts

Implements most of the functionality required by ValveSoftware#108

…ndalone

* commit 'dc47fdeceda1b58a563ce3a619b5a4b08a4f4493':
  Version 2.3.2 (sdk 1.4.18) - updating readme and openvr_api.cs
  Version 2.3.2 - Added optional fix for Unity 2019.2b / 2019.3a
  2.3.1b - Added no skeleton input fallback, bug fixes
  2.3b2 - removed settings.asset
  Version 2.3b - GC/Perf fixes. Seated Universe fix. Misc api updates