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/simple_config.py

@@ -4,7 +4,7 @@ import time
 import os
 import stat
 from decimal import Decimal
-from typing import Union, Optional, Dict, Sequence, Tuple, Any, Set, Callable
+from typing import Union, Optional, Dict, Sequence, Tuple, Any, Set, Callable, AbstractSet
 from numbers import Real
 from functools import cached_property
 
@@ -349,6 +349,10 @@ class SimpleConfig(Logger):
     def is_plugin_enabled(self, name: str) -> bool:
         return bool(self.get(f'plugins.{name}.enabled'))
 
+    def get_installed_plugins(self) -> AbstractSet[str]:
+        """Returns all plugin names registered in the config."""
+        return self.get('plugins', {}).keys()
+
     def enable_plugin(self, name: str):
         self.set_key(f'plugins.{name}.enabled', True, save=True)