Splits up wm_xr.c into multiple files in their own folder:
source/blender/windowmanager/xr. So this matches how the message bus and
gizmo code have their own folder and files.
This allows better structuring and should make the code scale better.
I rather do this early on than to wait until we end up with a single,
huge file.
Also improves a bit how data is prepared and updated for drawing.
