make checksum editable again for dependent objects

Author: LeftofZen

Gui/ViewModels/DatTypes/S5HeaderViewModel.cs

@@ -35,7 +35,7 @@ public S5HeaderViewModel(S5Header s5Header)
 		[Reactive, MaxLength(8)]
 		public string Name { get; set; }
 
-		[Reactive, ReadOnly(true)]
+		[Reactive]
 		public uint32_t Checksum { get; set; }
 
 		[Reactive]