Generalize per-platform support introducing PLATFORM_SUPPORTS_VR option

[jcfr]

Thank you, looks good. Nitpick: instead of using if(APPLE) at two different places, it would be nicer to set a variable like PLATFORM_SUPPORTS_VR once (set it to FALSE if APPLE, TRUE otherwise) and use that value at both places. I did not introduce this variable in the original commit because the check was done at one place and it was possible to explain that in a comment there.

Originally posted by @lassoan in #94 (comment)

[jcfr]

To enable the support for specific runtime, there is now the following CMake options:

• SlicerVirtualReality_HAS_OPENVR_SUPPORT
• SlicerVirtualReality_HAS_OPENXR_SUPPORT

By default, both OpenXR and OpenVR are now disabled on macOS:

• Disable OpenXR runtime on macOS and remove stub module #150

At runtime, there is also the XRRuntime property associated with the view node.