Split headers, etc.

[rpavlik]

This is a work in progress but nearly done.

Current state: produces these headers:
openxr.zip

cc @matthieucoquet - I incorporated some of your suggestions in here.

[rpavlik]

OK, this is a fairly substantial update, and really makes things better I think. Things are split into individual files to reduce include overhead and let people use e.g. the dispatch without too much else (the dispatch headers are actually entirely independent now), orthogonality is improved, doxygen is much better, usability bits smoothed out (I ported hello_xr and another project), etc.

Here are the generated headers:
openxr.zip

Here are the doxygen docs:
html.zip

[matthieucoquet]

I tried the headers (with some small changes related to what I mention in #27) and everything works well in my small app.

I just had to download the latest PowerShell to generate the headers. The version by default in Windows results in unclear errors. Maybe it should be mention in the Readme.

The macro to default the dispatcher are pretty nice.

[rpavlik]

Great, thanks for the review! I'll make a note in the README

[rpavlik]

I actually was able to just modify the script slightly so it works with the standard powershell in Windows 10 :)

[rpavlik]

OK, this has been updated slightly. (Main changes are fixing a flags bug, and doing a better job with Bool32.)

Also, for your review pleasure, I have ported hello_xr to the current state of this repo: https://github.com/rpavlik/hello_xr_hpp/tree/pr26-210112

Each commit is buildable so you can see the process I used.