plugins: structure plugin storage in wallet

store all plugin data by plugin name in a root dictionary `plugin_data`
inside the wallet db so that plugin data can get deleted again.
Prunes the data of plugins from the wallet db on wallet stop if the
plugin is not installed anymore.

electrum/json_db.py

@@ -189,6 +189,11 @@ class StoredDict(dict):
             self.db.add_patch({'op': 'remove', 'path': key_path(self.path, key)})
         return r
 
+    def setdefault(self, key, default = None, /):
+        if key not in self:
+            self.__setitem__(key, default)
+        return self[key]
+
 
 class StoredList(list):