Add a slate version field

core/src/libtx/slate.rs

@@ -92,6 +92,8 @@ pub struct Slate {
 	/// insert their public data here. For now, 0 is sender and 1
 	/// is receiver, though this will change for multi-party
 	pub participant_data: Vec<ParticipantData>,
+	/// Slate format version
+	pub version: Option<u64>,
 }
 
 impl Slate {
@@ -106,6 +108,7 @@ impl Slate {
 			height: 0,
 			lock_height: 0,
 			participant_data: vec![],
+			version:Some(1)
 		}
 	}