And check DistanceAboveTerrain instead of just vertical distance to upgrade
source.
This is necessary to avoid situations where an actor is technically on the right
vertical distance above/below ground, but not upgraded because it is located on a lower/higher terrain level than the upgrade source.
The following changes are made:
* Replaced the custom actor search with ProximityTriggers.
* Added a `Sticky` flag, which causes the ownership to not revert back to
the original owner when the captor leaves.
* Added trait documentation.
