Overhaul the Full Body Calibrator UI & calibration functionality

[AshtonSparx]

Is your feature request related to a problem? Please describe.

A lot of people are put off with how difficult the full body calibrator is, and in-accurate it can be. From the clunky UI, the old mannequin, optimize the spawning of it, and other things that people wish to be re-worked.

Relevant issues

#2949
#2588
#2518
Potentially #1598
#1120
#317

Describe the solution you'd like

A full overhaul of the FBT UI and how the calibration system works. A simple 1 time T-Pose with the hips not having the infamous "slouch" -- possibly not needing the entire calibration UI (Which currently needs spawn permissions to even use) unless you select like an "advance" option that than spawns out the usual (or re-worked) UI. The tweaks that was for-mentioned in the Relevant issues with the community feedback, and many other things to hopefully make it easier for you to implement things.

Describe alternatives you've considered

Possibly holding off till The UI Overhaul to get local calibration options so you don't need to spawn things into the world to do FB calibrations for the 1-Step T-Pose, while also addressing the Hip Pushing up issue & neck slouching.

Additional context

Here's some examples of the neck being slouched when you calibrate.

If you desire more pictures and examples of the issues with just a basic T-Pose calibration (This is with 3 trackers. Feet & Hip)

[FlameSoulis]

I can confirm this issue with even 5 trackers (elbows) and 7 (knees). The usual solution is to move the hips into unusual conditions or alter the height numbers. None of these are ideal and are a far cry compared to the competition.

In addition, the fact that avatar calibration and user calibration are separate means that it is harder to diagnose if the issue is with the user's calibration or the avatar's calibration. This has resulted in many cases where an incorrect user calibration ruins the calibration of owned avatars, resulting in a lot of wasted time and frustration.

[Mattyrogue]

There are a number of tangentially related things that can also affect the accuracy and efficacy of the Full Body Tracking setup that are either not properly explained, or poorly translated, these consist of the following;

• The number of Bones between the Hips, Spine and Chest; in many project files, there is a number of 'twist bones' often associated with Mechanim rigs. It's tangentically related to Issue #1601 (Twist bones), where the number of bones between these three parts of the IK can exacerbate the hunchback condition and affect overall accuracy.
• The placement of the Head Proxy Target; depending on if the Head Target is too far forward or too far back can exacerbate the 'hunchback effect' illustrated above, it's hard to really define it and infuriating that what is traditionally a fix reserved for clipping (moving the Head Target) can affect the feel of the whole avatar from an IK standpoint.
• The weights and constrains within the VRIK itself; generally these are fine for non-VR users, but even changing things such as the neck clamp values and chest lock (I forgot the actual value names it's been a while) can improve the IK even for non-fullbody users.

Naturally, I imagine each avatar is something of a unique setup and certain values will have to be tweaked accordingly, but even just going as far as to abstract the tracking data from the viewport position of the head proxy target would go a long way.