hw DeviceMgr: mostly switch away from xpubs for device pairing

- the DeviceMgr no longer uses xpubs to keep track of paired hw devices
- instead, introduce keystore.pairing_code(), based on soft_device_id
- xpubs are now only used in a single place when the actual pairing happens
- motivation is to allow pairing a single device with multiple generic
  output script descriptors, not just a single account-level xpub
- as a side-effect, we now allow pairing a device with multiple open
  windows simultaneously (if keystores have the same root fingerprint
  -- was already the case before if keystores had the same xpub)

electrum/base_wizard.py

@@ -622,7 +622,7 @@ class BaseWizard(Logger):
                 password = k.get_password_for_storage_encryption()
             except UserCancelled:
                 devmgr = self.plugins.device_manager
-                devmgr.unpair_xpub(k.xpub)
+                devmgr.unpair_pairing_code(k.pairing_code())
                 raise ChooseHwDeviceAgain()
             except BaseException as e:
                 self.logger.exception('')