Add String TypedTransfer::transferId() which returns null by default.

Author: nedtwigg

durian-swt/src/main/java/com/diffplug/common/swt/dnd/TypedTransfer.java

@@ -23,6 +23,11 @@
 
 /** A strongly-typed custom transfer type. */
 public abstract class TypedTransfer<T> extends ByteArrayTransfer {
+	/** The transfer id. */
+	public @Nullable String transferId() {
+		return null;
+	}
+
 	public abstract T getValue(DropTargetEvent e);
 
 	public abstract void setValue(DragSourceEvent e, T value);