Rework mass properties

[janhohenheim]

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Context Quoting @Jondolf:

Mass properties are one of the messiest user-facing parts, and we really need to rework them a lot

One thing that needs to be more accessible is getting the density of a collider / rigid body.

[Jondolf]

I made a write-up laying out a lot of the current problems, along with a proposal for overhauling mass properties: #499