Improve Asset.parameters

[fabienrohrer]

For now, the customizable parameters are TinkerbotsAxle.length (actually not implemented), TinkerbotsCubiePyramid.color and TinkerbotsCubieTriangle.color. In Webots, they are all string enumerations, and affects the Shape (color or shape).

In the contrary of translation or rotation where the THREEjs <-> Webots link is direct, this kind of links should be coded somewhere. For now it's hard-coded here: https://github.com/omichel/robot-designer/blob/master/app/view/mediator/part_mediator.js#L78

To solve this, we should either provide a generic way to write these links, or to have several x3d files (e.g. one x3d file per parameter set).

[fabienrohrer]

Moreover, initializing a parameter affecting a mesh in the part constructor is not straightforward, because the representation may be not loaded (x3d files are loaded asynchronously). This is currently the case when undo/redo the deletion of a colored mesh.