This change makes it so the list interface in the properties panels looks
closer to things like shape keys, vertex groups and so on: there are two
buttons to add selected objects to the collection and remove active item
from the collection, as well as the "extra" drop down menu.
The add operator adds selected objects to the light linking collection
using the Include policy. For the light linking it means that the objects
are added as receivers that receive the light, and for the shadow linking
it means that objects are added as blockers which cast shadow from the
light.
The communication of the active list element is done via context property
similar to how it was done before. The difference is that these properties
are set on a parent of the list layout, which makes it so they are inherited
by the layout hierarchy needed to place the Remove button.
Pull Request: https://projects.blender.org/blender/blender/pulls/112713
015bc7ca4d