[Feature] Export lights for collada world exporter

[ddengster]

Desired behavior

Enable capabilities to export lights into the .dae format along with the collada world exporter.

Implementation suggestion

Iterate through ecm.foreach for each light and 'add' each light to the dae file.

Additional context

This helps ease the lightmapping process. By maintaining the consistency of lights between ignition and blender, the lighting artist need not tweak lights to sync to the correct positions; and the latest stage of modification will give the final light positions/orientations. This also means ensuring PR #895 exports the light's position/orientations correctly in sdf format.

I'll be working on this feature on this branch. Let me know if there's an alternative.