Use IDs instead of indices for mission data

[cxong]

The mission data currently uses index numbers. This makes the campaign editor flaky: delete an item, and all other references to that item now point to a different item. E.g. objectives, characters. Instead, use an ID number unrelated to their position in the arrays. This includes:

• Characters (enemies, kill/rescue objectives, static characters)
• Objectives (static objectives)

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