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BGE physics: get/set linear and angular damping

This patch adds the following R/W properties and method to `KX_GameObject`:

- `linearDamping` -- get/set linear damping
- `angluarDamping` -- get/set angular damping
- `setDamping(linear, angular)` -- set both simultaneously

These allow runtime changes to the same properties that are accessible at design time in Blender's UI via `game.damping` and `game.rotation_damping`. The names of the properties were chosen to mirror the internal names of the BGE physics engine, as these are (AFAIK) also the commonly used names in physics literature.

Reviewers: campbellbarton

Projects: #game_physics

Differential Revision: https://developer.blender.org/D936