Request: trap ctrl-c in hello_xr, shutdown openxr cleanly

As well as being a 'hello, world' demo app, hello_xr is useful as a quick-launching test app when working on API layers (in my case, https://github.com/fredemmott/OpenKneeboard). For this case, it would be useful to make it a bit more 'well behaved'.

I tried using a `std::unique_ptr<>` to hold my resources, and calling `xrDestroyFoo()` from my destructor

This makes `hello_xr` hang forever on ctrl-c, inside my calls to `xrDestroySwapchain` or `xrDestroySpace`; this seems to be because the openxr resources/functions have already been unloaded before my API layer's DLL is unloaded, and in turn, before my static `std::unique_ptr`'s destructor is called.

To workaround, I'm using a raw pointer, and `delete`'ing from my `xrDestroySession` override, which effectively leaks the resources on shutdown; this isn't directly a problem, but in general, it's good to be able to use RAII instead of raw pointers where possible.

This could be done via `signal()` on unix-like platforms, or `SetConsoleCtrlHandler()` on windows

Start in the hello_xr shutdown path and compare the requested Unix signal() and Windows SetConsoleCtrlHandler() approaches. Define completion as ctrl-c allowing OpenXR resources such as swapchains and spaces to be destroyed cleanly without hanging, while supporting the relevant platforms.

Written by the indexing model from the issue text.