Migrate hard-coded position constraints to assets.json

[fabienrohrer]

For now the position constraints are hard-coded here:

https://github.com/omichel/robot-designer/blob/master/app/view/handle.js#L109

It would be good to migrate them to asset/assets.json.

[fabienrohrer]

This is probably the most urgent remaining core issue to be able to build any Tinkerbots model from the user guide.

[fabienrohrer]

It should be possible to rotate the TinkerbotsCubieBoxWithRoundSlots.axisSlot.