Blender Git Loki

Geometry Nodes: Enable exposing object and collection sockets

This patch allows connecting wires for object and collection socket
types to the "Group Input" node, which exposes them to be adjusted
in the modifier.

Thanks to @angavrilov's recent work in rB8964c02348f6, it is now
possible to edit pointer IDProperties in the interface when they are
drawn with `uiItemPointerR`.

This patch is composed of a few changes:
- Add code to create pointer properties in the modifier settings for
object and collection sockets, and also to draw them in the UI.
- Also search through the modifier's `IDProperty` settings to find IDs
used by the modifier.
- Change the setting's UI layout to support the change.

Differential Revision: https://developer.blender.org/D10056