Resource Gathering AI Improvements

[MNoya]

• Add idle position-differential thinker while not actively chopping wood, to determine that the builder has stopped while trying to reach a tree and should try to move around choosing another close to it
• Add more data to trees, such as the list of builders currently working on it (instead of a single one) and access level (an unreachable tree has 0 while a solitary tree has 8 or even more peons that could work on it)
• Make a "Tree Island" system that keeps references to trees in certain zone, to improve the efficiency of decisions and tree finding
• Ordering multiple gatherers to grab from a tree:
  • should always go towards that island
  • assign more than one builder to each tree when there aren't enough reachable trees to gather from
• Try to make these scripts library-able
  • order filtering should be transparent
  • provide a settings file
• DeterminePathableTrees should save to file per map while IsInToolsMode(), load to save processing