Inherit the parent's tags when creating a new subtask.

If a parent is provided during task creation, all tags
of the parent are added to the newly created task.

Fixes GitHub issue #1078

GTG/core/tasks.py

@@ -763,6 +763,11 @@ def new(self, title: str = None, parent: UUID = None) -> Task:
 
         self.add(task, parent)
 
+        # add the tags of the parent
+        if parent is not None:
+            for tag in self.lookup[parent].tags:
+                task.add_tag(tag)
+
         return task