Simplify the GPUParticles3D editor gizmo to improve readability

[Calinou]

• Remove handles as users should use the Generate Visibility AABB option whenever possible, so manual resizing isn't needed often. (For particles that move quickly and have Local Coords disabled, Extra Cull Margin can be increased in the GeometryInstance3D properties in the inspector.)
• Remove translucent filled part of the gizmo to be more consistent with GeometryInstance3D's recently introduced Custom AABB gizmo.
• Remove center marker as it can visually obstruct particles.

This paves the way for future emission shape gizmos which will feature handles.

• See Hide the GPUParticles3D Visibility AABB gizmo by default godot-proposals#9591.

Preview

GPUParticles3D

GeometryInstance3D Custom AABB (for comparison)

[akien-mga]

Makes sense to me, I always found this giant orange AABB annoying.

[akien-mga]

Thanks!

[GeminiSquishGames]

This is a really neat idea for making the AABB, but I was playing with it and noticed particle attractors and colliders only work on particles if they are within the emitter's AABB. Attractors and colliders can push the particles outside of the AABB and also with any non-deterministic behaviors like Time Randomness, Lifetime randomness, and turbulence, particles will sometimes be outside of the calculated AABB. I assume it's only precalculating one emissive cycle and not doing an extra prediction for the randomness. I'm guessing one justs extends the margin to compensate. It makes a good estimate, the majority of them fall within the AABB and only dynamic or moving colliders and attractors are problematic (at least the box moves with the emitter and can be changed at runtime), and without the handles a little annoying to calculate, but I may just be nitpicking at that point if I'm not entirely. A lot of things I would love to see runtime helpers with, but I know runtime calculations and particle detection is hard since it's done visually on the GPU and the positions of particles aren't exposed in high level scripting for sake of performance in many cases like this, such as the MSDF calculated particle colliders. I doubt too many people will notice, but just in case this is important enough to consider.