Add AnimatableBody inherited from StaticBody for moving platforms

Instead of having a physics node named Static that can be either Static
or Kinematic, AnimatableBody is added again as a separate node:
-Inherited from StaticBody to make its usage clearer
-Still separated from CharacterBody to make its usage more focused

Properly implemented constant velocity for kinematic bodies in godot
physics servers (induced velocity without actually moving).

Also updated description for the different physics nodes to make their
usage clearer.

editor/icons/AnimatableBody2D.svg

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