add persistence

[laurentopia]

• stores mesh assets neatly alongside root sharedMesh, inside a scriptableobject that also stores parameters (noodled from Unity's implementation of V-HACD)
• instead of a hierarchy of children (which increases the cost of hieararchy traversal etc...), stores all meshcolliders on the same gameobject and hide them

[eliphatfs]

Hi laurentopia, thank you for contributing to CoACD! I looked into the code and I like the idea of storing meshes on the same GameObject and persistence of mesh data automatically. However, the current dependency on SaintsField.Playa may cause confusion on the usage without reading the code or instructions on the editor extensions, and the EditorUtility call would make the code fail to compile on build. I would like to accept the PR but also make some changes after.