Allow declaration of war via air mission

[Soar_Slitherine]

Right now, air units can't be used to initiate a war against another major regime, only ground units can. This can be particularly awkward if you want to launch air attacks against a regime you don't actually have a land border with (because they're fighting your ally that needs support, or there's a danger of them running away with the game, etc). Air forces can be used to wage strategic air war independent of ground forces, so they should also be able to initiate war independently.