Mission dependencies

[WsevoL0D]

Add CompleteToUnlock(for instance) property for campaign (or even standalone) missions, with collection of strings - names of missions player must accomplish before mission that hosts the property become available, e.g.

Allied Campaign:
	mission 1:
		./mods/ra/maps/allies-01
	mission 2:
		./mods/ra/maps/allies-02
		CompleteToUnlock: mission 1

Mission 2 stays locked until mission 1 completed and so on.

This intended to enhance nostalgic experience. Also this could be handy for modders, who creating their own campaigns.

[abcdefg30]

Also see #4492.

[WsevoL0D]

Yes. I think this both features will work nice in conjunction.