Add ReloadAmmoPool and adapt AmmoPool

Refactored and simplified Rearm activity.
Uses local Reload now.

Removed AmmoPool.SelfReloads.

OpenRA.Mods.Common/Activities/Air/HeliReturnToBase.cs

@@ -109,7 +109,7 @@ namespace OpenRA.Mods.Common.Activities
 				return true;
 
 			return heli.Info.RearmBuildings.Contains(dest.Info.Name) && self.TraitsImplementing<AmmoPool>()
-					.Any(p => !p.Info.SelfReloads && !p.FullAmmo());
+					.Any(p => !p.SelfReloads && !p.FullAmmo());
 		}
 	}
 }