Implement_add_hookspec_dict in add_hookspecs (#34)

* Implement_add_hookspec_dict in add_hookspecs

Currently, register calls _register_dict if namespace is a dict. It seems like this functionality was also intended for add_hookspecs, but _add_hookspec_dict is never called anywhere in either napari or napari_plugin_engine. The four functions' git blame patterns lead me to believe this was an accident.

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napari_plugin_engine/manager.py

@@ -521,6 +521,10 @@ def add_hookspecs(self, namespace: Any):
 
         Functions are recognized if they have been decorated accordingly.
         """
+
+        if isinstance(namespace, dict):
+            return self._add_hookspec_dict(namespace)
+
         names = []
         for name in dir(namespace):
             method = getattr(namespace, name)