Support start on the EAST side

[LordGolias]

Currently the mod only supports starting from WEST. This limits replayability. Let us generalize the code we have to support FIA on either EAST or WEST, and obviously NATO/CSAT etc. on opposite fields.

I am working on this for the past week. So far:

• [x] base code no longer depends on the player side
• [x] selection of units for each spawn no longer depends on side nor faction
• [x] all units/vehicles from AAF/CSAT/NATO are on the same interface
• [x] make FIA use same interface as above
• [x] make player start as CIV, and only assign a side once the commander selects the setting (or loads a game)

[LordGolias]

The PR #70 solves this. It needs testing and bug removal.