Clean up UI of ReflectionProbe

[lander-vr]

This PR cleans up some of the UI of ReflectionProbes.
Proposal: godotengine/godot-proposals#11269

• Removed corner to center-offset lines
• Only show center-offset representation cross when center-offset actually has an offset
  • I would prefer to use an icon that shows up when the probe is selected, instead of the cross, but it doesn't look like add_unscaled_billboard supports offsets.
• Removed handles for the center-offset
• Significantly reduced opacity on solid box

Still missing from that proposal:
Improved bounding box handles feedback and depth, however that should be an improvement for all objects using handles, for which a separate PR makes more sense. (#60474)

Master	this PR

[Calinou]

Tested locally, it works as expected.

We should also decrease VoxelGI's gizmo opacity, as it's now much more opaque than ReflectionProbe's. Can you look into this as well?

[lander-vr]

Tested locally, it works as expected.

Is there anything else I should do to get this approved? :)

[Calinou]

Is there anything else I should do to get this approved? :)

Could you also reduce the VoxelGI gizmo's opacity? It makes sense to do this in the same PR since they're closely related.

[lander-vr]

Could you also reduce the VoxelGI gizmo's opacity? It makes sense to do this in the same PR since they're closely related.

Already did that in a separate PR, thought it would make more sense since this PR includes a bit more than just adjusting the opacity of the materials.
#99969

[Repiteo]

Thanks!