Decouple add from nextFreeIndex in IdMap

src/main/kotlin/ru/dbotthepony/kstarbound/collect/IdMap.kt

@@ -19,7 +19,7 @@ class IdMap<T : Any>(val min: Int = 0, val max: Int = Int.MAX_VALUE, private val
 		return nextIndex
 	}
 
-	fun add(value: T): Int {
+	fun nextFreeIndex(): Int {
 		var i = 0
 		var index = next()
 
@@ -41,6 +41,11 @@ class IdMap<T : Any>(val min: Int = 0, val max: Int = Int.MAX_VALUE, private val
 			}
 		}
 
+		return index
+	}
+
+	fun add(value: T): Int {
+		val index = nextFreeIndex()
 		map[index] = value
 		return index
 	}