Defer the repainting of the node in the tree

The previous change actually broke some of the tests in test_disabled.py --
well actually it flat out broke one and caused all the others to run *very*
slowly. No clue why though.

But thinking about this some more, it does feel like delaying the refresh of
the node makes more sense.

src/textual/widgets/_tree.py

@@ -307,7 +307,7 @@ class TreeNode(Generic[TreeDataType]):
         self._updates += 1
         text_label = self._tree.process_label(label)
         self._label = text_label
-        self._tree._refresh_node(self)
+        self._tree.call_later(self._tree._refresh_node, self)
 
     def add(
         self,