Groupbox: Set spacing to margin_x in configure

widget_defaults is not picked up in __init__, which means that after a
config reload we get different spacing (margin_x will be different).

libqtile/widget/groupbox.py

@@ -243,8 +243,6 @@ class GroupBox(_GroupBase):
     def __init__(self, **config):
         _GroupBase.__init__(self, **config)
         self.add_defaults(GroupBox.defaults)
-        if self.spacing is None:
-            self.spacing = self.margin_x
         self.clicked = None
         self.click = None
 
@@ -258,6 +256,11 @@ def __init__(self, **config):
             )
         self.add_callbacks(default_callbacks)
 
+    def _configure(self, qtile, bar):
+        _GroupBase._configure(self, qtile, bar)
+        if self.spacing is None:
+            self.spacing = self.margin_x
+
     @property
     def groups(self):
         """