Blender Git Loki

Geometry Nodes: Attribute search drop-down

This commit adds a search for existing attributes when you click
on an attribute field. This is useful because otherwise you have
to remember which attributes should be available at each node in
the tree.

The fundamental complication is that this information is not
accessible statically. So the search data is only a cache from
the previous node tree evaluation. The information is added
with `BKE_nodetree_attribute_hint_add`, currently for every
input geometry socket for a single node.

This is only an initial implementation, and later versions will
expose the data type and domain of the attributes.

Differential Revision: https://developer.blender.org/D10519