fixed a bug where a dynamic item would not get saved/loaded at all

src/net/torvald/terrarum/savegame/SimpleFileSystem.kt

@@ -1,5 +1,6 @@
 package net.torvald.terrarum.savegame
 
+import java.io.File
 import java.nio.charset.Charset
 
 /**
@@ -9,4 +10,5 @@ interface SimpleFileSystem {
     fun getEntry(id: EntryID): DiskEntry?
     fun getFile(id: EntryID): EntryFile?
     fun getDiskName(charset: Charset): String
+    fun getBackingFile(): File?
 }