changing mass to or from +inf does not affect mobility

[cburschka]

Giving a node infinite mass in the layout will result in Box2DSimulation creating a static body.

Changing the mass of an existing node between non-infinite values works as expected, but changing the mass to infinity does not make a previously mobile body static, nor vice versa.

Right now, this means any infinite mass must be assigned before the node is added to the layout in order to work.