Model preview in flatland_viz (fixes #15)

[josephduchesne]

• When placing a model, flatland_viz spawn model tool now loads and renders a preview of the actual model shape (currently just in mono color)
• The previous placeholder is no longer used, so its model and model specific quirks (90 degree offset) have been removed.
• This does not take into account Joints (welds especially) since that would add a lot of complexity. This could be added in the future, but would likely require either instantiating Box2d in flatland_viz, emulating the joint constraints, or passing off loading to flatland_server somehow.

Fixes #15