Expose DynamicRigidBody velocity properties

Expose the synchronized linear and angular velocities of a dynamic
rigid body as read-only QML properties. Direct access to simulated
body motion is useful for physics calculations and diagnostics in QML
without maintaining duplicate state.

[ChangeLog][DynamicRigidBody] Added read-only linearVelocity and
angularVelocity properties.

Change-Id: I579377d31cccd8a62fecd608b34f45ffcfcaa76f
Reviewed-by: Jonas Karlsson <jonas.karlsson@qt.io>