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/keystore.py

@@ -915,6 +915,12 @@ class Hardware_KeyStore(Xpub, KeyStore):
             self.soft_device_id = client.get_soft_device_id()
             self.is_requesting_to_be_rewritten_to_wallet_file = True
 
+    def pairing_code(self) -> Optional[str]:
+        """Used by the DeviceMgr to keep track of paired hw devices."""
+        if not self.soft_device_id:
+            return None
+        return f"{self.plugin.name}/{self.soft_device_id}"
+
 
 KeyStoreWithMPK = Union[KeyStore, MasterPublicKeyMixin]  # intersection really...
 AddressIndexGeneric = Union[Sequence[int], str]  # can be hex pubkey str