Update visualize collisions to support 1.8 composition

[jennuine]

In Citadel & Dome, which uses SDFormat 1.7, when a sdf model contained collisions, then right-clicking to View > Collisions would display all collisions of the model in orange. In Edifice, this no longer works correctly if the model has nested models.

The current implementation (see #531 for details) checks links of the model for collision entities but assumes that links will be direct children of the model entity. To support SDFormat 1.8, the implementation needs to consider nested models.

To test this, run ign gazebo nested_model.sdf, then in the entity tree right-click model_00 > View > Collisions and only the sphere will be highlighted in orange.