Bump version to v1.0, update README and credits

This commit:

* updates version to v1.0,
* updates README.md to note that Q3VR is out of the beta,
* updates credits to include idSoftware/Q3A
* adds "credits" on main screen (reference that this port is based on
  ioquake3 and Quake3Quest)

[VR] Initial support for VR via OpenXR

This commit adds initial support for playing Quake 3 Arena in VR using
OpenXR API. This work was initially based on Quake3Quest project which
was rebased on top of newest `ioquake3` idTech3 fork, adjusted for PCVR
(including rewriting big chunks of OpenXR-related code) with a lot of
bugs fixed and many features and QoL improvements added along the way.

Currently building and playing is supported only on Windows (with MSVC
or Mingw).