Always copy config objects

If we mutate the value we get from the config, we want to make sure the value in
the config always stays the same (especially when it's the default!).

qutebrowser/config/config.py

@@ -412,7 +412,10 @@ def get_obj(self, name, *, mutable=True):
         If mutable=True is set, watch the returned object for mutations.
         """
         opt = self.get_opt(name)
-        obj = self._values.get(name, opt.default)
+        # We always return a copy of the value stored internally, so the
+        # internal value can never be changed by mutating the object returned.
+        obj = copy.deepcopy(self._values.get(name, opt.default))
+        # Then we watch the returned object for changes.
         if isinstance(obj, (dict, list)):
             if mutable:
                 self._mutables.append((name, copy.deepcopy(obj), obj))