Plan for merging hero and NPC walkabout layers

Currently, Heros and NPCs are in separate layes, each of which is a child layer of the walkabout layer. Heroes are automatically sorted by Y value, and NPCs go unsorted. The order of the NPC and hero layers is customizable via a general map data option, and it matters for old games that use NPCs as overhead objects (from back before map layers existed)

• In general map data, add a third state for "Walkabout Layering". In addition to "Heroes over NPCs" and "NPCs over Heroes", add "Together"
• "Together" should be the default for new maps.
• In Together mode, the hero and NPC slices will be parented directly in the walkabout layer, and the hero layer and npc layer will not exist at all.
• In Together mode, the walkabout layer will be automatically set to sort by Y value (although the user can override that in their map autorun script)