Overlay rewrite

[Frozenreflex]

Replaces the C++ based SteamVR-only overlay with a new overlay written in C#, using Godot, that supports multiple backend modes (SteamVR overlay, OpenXR standalone) and is cross-platform. The overlay resides here, and will get pulled in when built with Github Actions.
The trainer has similarly been moved into it's own repo

• Finish moving the trainer into it's own repo
• Build trainer with Github Actions
• Pull trainer artifact in when Baballonia is built with Github Actions
• Add scripts to pull in both repos locally, so developers don't need to manually download artifacts

also hides the preferred backend behind an "advanced options" toggle to please sierra and whoever else was complaining that it was the worst thing in the world

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ RedInJector
❌ Frozenreflex
_{You have signed the CLA already but the status is still pending? Let us recheck it.}