Allow array of identifiers in id node of objects loaded by generic factory

[ZhilkinSerg]

Summary

SUMMARY: None

Purpose of change

• Clean up some code of generic factory (make node naming consistent across various object types).
• Allow array of identifiers in id node of objects loaded by generic factory (can be useful to reduce cluttering of overmap terrain definitions by combining entries with same settings). This will allow to replace the following:

  {
    "id": "urban_4_6",
    "type": "overmap_terrain",
    "name": "house",
    "copy-from": "generic_city_building_no_sidewalk",
    "color": "light_green",
    "see_cost": 10
  },
  {
    "id": "urban_4_7",
    "type": "overmap_terrain",
    "name": "house",
    "copy-from": "generic_city_building_no_sidewalk",
    "color": "light_green",
    "see_cost": 10
  },
  {
    "id": "urban_4_8",
    "type": "overmap_terrain",
    "name": "house",
    "copy-from": "generic_city_building_no_sidewalk",
    "color": "light_green",
    "see_cost": 10
  }

with the following:

  {
    "id": [ "urban_4_6", "urban_4_7", "urban_4_8" ] ,
    "type": "overmap_terrain",
    "name": "house",
    "copy-from": "generic_city_building_no_sidewalk",
    "color": "light_green",
    "see_cost": 10
  },

Describe the solution

See commit messages.

Testing

Make sure it compiles, tests are running cleanly and there are no loading errors.

Additional context

I will add JSON changes for overmap terrains in follow-up PR(s).