qt plugins dialog: fix caching "settings" button

shesek reported on IRC:
> the button widget for opening plugins configuration gets cached in `settings_widgets`
> even after the plugin is disabled and re-enabled, which causes it to call `settings_dialog()`
> on the previous plugin instance that got unloaded instead of the new one.

electrum/plugin.py

@@ -43,6 +43,7 @@ from .logging import get_logger, Logger
 if TYPE_CHECKING:
     from .plugins.hw_wallet import HW_PluginBase, HardwareClientBase, HardwareHandlerBase
     from .keystore import Hardware_KeyStore
+    from .wallet import Abstract_Wallet
 
 
 _logger = get_logger(__name__)
@@ -107,7 +108,7 @@ class Plugins(DaemonThread):
     def count(self):
         return len(self.plugins)
 
-    def load_plugin(self, name):
+    def load_plugin(self, name) -> 'BasePlugin':
         if name in self.plugins:
             return self.plugins[name]
         full_name = f'electrum.plugins.{name}.{self.gui_name}'
@@ -129,14 +130,14 @@ class Plugins(DaemonThread):
     def close_plugin(self, plugin):
         self.remove_jobs(plugin.thread_jobs())
 
-    def enable(self, name):
+    def enable(self, name: str) -> 'BasePlugin':
         self.config.set_key('use_' + name, True, True)
         p = self.get(name)
         if p:
             return p
         return self.load_plugin(name)
 
-    def disable(self, name):
+    def disable(self, name: str) -> None:
         self.config.set_key('use_' + name, False, True)
         p = self.get(name)
         if not p:
@@ -145,11 +146,11 @@ class Plugins(DaemonThread):
         p.close()
         self.logger.info(f"closed {name}")
 
-    def toggle(self, name):
+    def toggle(self, name: str) -> Optional['BasePlugin']:
         p = self.get(name)
         return self.disable(name) if p else self.enable(name)
 
-    def is_available(self, name, w):
+    def is_available(self, name: str, wallet: 'Abstract_Wallet') -> bool:
         d = self.descriptions.get(name)
         if not d:
             return False
@@ -161,7 +162,7 @@ class Plugins(DaemonThread):
                 self.logger.warning(f'Plugin {name} unavailable: {repr(e)}')
                 return False
         requires = d.get('requires_wallet_type', [])
-        return not requires or w.wallet_type in requires
+        return not requires or wallet.wallet_type in requires
 
     def get_hardware_support(self):
         out = []
@@ -270,7 +271,7 @@ class BasePlugin(Logger):
     def on_close(self):
         pass
 
-    def requires_settings(self):
+    def requires_settings(self) -> bool:
         return False
 
     def thread_jobs(self):
@@ -285,8 +286,11 @@ class BasePlugin(Logger):
     def can_user_disable(self):
         return True
 
-    def settings_dialog(self):
-        pass
+    def settings_widget(self, window):
+        raise NotImplementedError()
+
+    def settings_dialog(self, window):
+        raise NotImplementedError()
 
 
 class DeviceUnpairableError(UserFacingException): pass