Rigidbody2d should allow the node to sleep in Character mode if "can_sleep" option is checked

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Godot version:
3.0.2

OS/device including version:
Windows 10

Issue description:
Currently it seems like the only way to stop a rigidbody2d from rotating when falling is to put it in 'Character' mode and that's fine, but the moment you put it into character mode the ability to sleep doesn't exist. I found that this is normal in #7996

What would you like to happen instead?
I would love it if the "can_sleep" option is set to true then in character mode the sleeping works how you would expect it to just without the rotation when gravity is involved.

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Or maybe add a new mode with this functionality?

[ShyRed]

You can prevent a RigidBody2D from rotating by implementing the _integrate_forces method ( see http://docs.godotengine.org/en/3.0/classes/class_rigidbody2d.html#class-rigidbody2d-integrate-forces ) and setting the state parameter's angular_velocity to zero and/or even set the transform's rotation to a constant value.

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@ShyRed closing for now. That alternative sort of works, but not really