QgsChunkedEntity gets a capability to update an existing sub-entity for a node
by loading the node again, creating a new 3D entity, and then discarding the old
entity in favor of the new one.
QgsPointCloudLayer gets a new signal - emitted whenever a particular node's
attribute values get changed.
These two are combined for point cloud layer's 3D chunked entity: whenever there
are edits to the point cloud data, instead of doing big hard reload of the whole
point cloud, we only update entities of the few nodes that were modified.
This greatly improves the user experience of point cloud editing (no "flashing"
of the 3D view while loading everything from scratch), and it improves how quickly
the changes can be seen.
