hww: stop keystore.thread when closing wallet

previously left running? Qt on macOS was complaining:
```
QThread: Destroyed while thread is still running
Abort trap: 6
```

electrum/plugins/hw_wallet/plugin.py

@@ -81,6 +81,8 @@ class HW_PluginBase(BasePlugin):
         for keystore in wallet.get_keystores():
             if isinstance(keystore, self.keystore_class):
                 self.device_manager().unpair_xpub(keystore.xpub)
+                if keystore.thread:
+                    keystore.thread.stop()
 
     def scan_and_create_client_for_device(self, *, device_id: str, wizard: 'BaseWizard') -> 'HardwareClientBase':
         devmgr = self.device_manager()