Added a stray behavior and a new behavior node for random movements.
Make the new SetTargetToNearbyBlockNode node available in the UI.
Don't fail with an exception if there is no target to search a path to.
MinionMoveSystem: Properly detect loading and unloading of entities.
Let SetTargetToNearbyBlock properly report failures.

Even if the failures have a technical background, it helps
debbuging if they get reported.
Fix broken behaviors after loading a world.
Make the stray behavior play walk and idle animations.