Add Bounds property binder and organize inspector

[apkd]

This commit adds a bounds property to the SkinnedMeshBinder, allowing you to easily deal with a situation where the character moves away from the point of origin. It allows you to select an exposed AABox graph property that you can easily connect to the effect's bounds input. There's an additional slider for expanding the bounds (since usually the VFX bounds should be larger than the mesh bounds). I also organized the inspector a little because I found it confusing at first sight.

Usage of the bound property looks like this:

[keijiro]

Thanks for the PR, but I'd like to close all the PRs because now VFX Graph officially supports Skinned Mesh Sampling.