Configurable lights rework
Customize additional lights. Motivated by the OSPRay renderer, but benefits OpenGL too. A dockable LightInspector allows adding lights, and creates a pqProxyWidget for each. Customized buttons handle removing a light, and syncing the light's position to the camera. Tracing and undo/redo are handled.
More features are on the TODO list, but this basic implementation is usable.
CC @demarle @utkarsh.ayachit @cory-quammen
Should we squash commits?