Fix nebula-related infinite recursion.

[DaloLorn]

• [x] Remove the existing nebula territory system.
• [x] After nebulae are created, compute a list of all connected nebulae to bring them into a unified "macronebula" for use with territories. Also compute a list of all systems adjacent to the macronebula.
• [x] When a system adjacent to a macronebula changes ownership, check if the macronebula should be added/removed from an empire's territory by checking whether other adjacent systems are owned by that empire.
• [x] If a new system is created adjacent to a macronebula, add it to the macronebula's adjacency list.

[DaloLorn]

Okay, writing this down for further reference:

• [x] Create Macronebula class. Store all instances of Macronebula in a global array (TODO: figure out how to do that).
• [x] Iterate through all systems in a galaxy at the end of MapGeneration::generate(). If they have the nebula flag (TODO: move NEBULA_FLAG constant from objects/Territory.as to map_generation.as):
  • [x] If they are not in a macronebula, create a new macronebula and add all adjacent nebulae to it. Also add all adjacent non-nebulae to its edges.
  • [x] If they are in a macronebula, do nothing.
• [x] Override system creation scripts outside map_generation.as (things like planet generators or the Expanse) to check for nearby macronebulae. Also, check if the new system is itself a nebula, if applicable.
• [x] Add a Macronebula pointer to all Regions so that nebulae know which macronebula they belong to. (In hindsight, this may mean I don't need the global array after all.)

Macronebula should store two arrays: one pointing at all of its component nebulae, and one pointing at all systems adjacent to it. The component nebulae should, as mentioned above, point back to their Macronebula instance for ease of access.

[DaloLorn]

This is now done. Nebulae can be used in the game again.