Add Mass Distribution, Deactivation, Solver inspector property groups

[aaronfranke]

RigidBody(2D/3D) is a complex class with many properties, so I think it's worth making some sections.

• The Mass Distribution section has properties for how mass is distributed: center of mass and inertia.
• The Deactivation section has properties for sleeping, locking, and freezing.
• The Solver section has properties for how the physics simulation solves/calculates things, like integration, continuous collision, and if the contact monitor is enabled.
• The mass, physics material, and gravity scale properties stay top-level since they are common to use and easy to understand for novice users.

When expanded, it looks like this:

When collapsed, it looks like this, very tidy:

[akien-mga]

Seems fine to me.

[akien-mga]

Thanks!