Scaling tool

[chapulina]

The Transform Control tool lets users translate and rotate entities in simulation. We should also add a scaling tool to that widget.

Ignition Rendering already supports scaling all types of visuals. The main missing piece right now is physics, see https://github.com/ignitionrobotics/ign-physics/issues/65. Once that issue is solved, we can:

• Add a scaling command to UserCommands
• Add a scaling button to TransformControl
• We may need to add scaling "heads" to Ignition Rendering's AxisVisual or some other visual