Actor reference ID on debug window, RefID is now Int, added Japanese on help page

Former-commit-id: 3519b8746d611c4badd65f01644ba05e41d10d04
Former-commit-id: 45bb407c2d932e5d70aab9c9eb4f3ef55ce68d27

src/net/torvald/terrarum/gameitem/InventoryItem.kt

@@ -12,7 +12,7 @@ interface InventoryItem {
      * 4097-32767: Various items
      * >=32768: Actor RefID
      */
-    var itemID: Long
+    var itemID: Int
 
     /**
      * Weight of the item, Float

src/net/torvald/terrarum/gameitem/TileAsItem.kt

@@ -8,12 +8,12 @@ import org.newdawn.slick.GameContainer
  */
 class TileAsItem(tileNum: Int) : InventoryItem {
 
-    override var itemID: Long = -1
+    override var itemID: Int = 0
     override var mass: Float = 0f
     override var scale: Float = 1f
 
     init {
-        itemID = tileNum.toLong()
+        itemID = tileNum
         mass = TilePropCodex.getProp(tileNum).density / 1000f
     }