Prevent reparenting a tag to itself

Sidebar.check_parent now checks if the drop target is the same
tag we are moving.

GTG/gtk/browser/sidebar.py

@@ -591,7 +591,10 @@ def drag_end(self, source, drag, unused):
 
 
     def check_parent(self, value, target) -> bool:
-        """Check to parenting a parent to its own children"""
+        """Check for parenting a tag to its own descendant or to itself."""
+
+        if value == target:
+            return False
 
         item = target
         while item.parent: