Discrepancy between physics joints behavior and editor gizmo

[pouleyKetchoupp]

Godot version:
stable 3.2.2
Also reproduced in master cf03f90

OS/device including version:
Windows 10

Issue description:
Hinge and Cone joint editor gizmos don't show reliably how the joints behave in physics, which makes them extremely difficult to setup.

PhysicalBone joints also have similar issues.

Steps to reproduce:
See minimal repro projects below. The following steps refer to the different test case scenarios from these projects.
Run the scene to compare the joints behavior with the gizmos in the editor.

Case 1: Hinge at origin (red color)
The gizmo doesn't show up at all in the editor when the joint is at the same position as its child rigid body.

Case 2: Rotated Hinge with limits (green color)
The gizmo shows 180° of freedom at the back, the child rigid body rotates in a 180° range on the right side instead.
(can be reproduced with both Bullet and GodotPhysics)

Case 3: ConeTwist at origin with swing 45° & twist 180° (blue color)
The gizmo shows only a vertical line, but the child rigid body can rotate with 180° range around X axis (swing axis).
(can be reproduced with both Bullet and GodotPhysics)

Case 4: ConeTwist with swing 20° & twist 180° (yellow color)
The gizmo shows a cone with twist on Y axis and swing on X/Z axes according to the settings, but the child rigid body can swing with 180° range around X axis and the twist seems limited on Y axis (looks like axes are inverted).
(can be reproduced with both Bullet and GodotPhysics)

Case 3 & 4: In both cases, the child rigid body is fixed to (0, 0, 0) instead of using the offset from the joint when using Bullet (can't reproduce with GodotPhysics).

Minimal reproduction project:
3.2:
joint-gizmo-3.2.zip

4.0:
joint-gizmo.zip

[pouleyKetchoupp]

CC @AndreaCatania