keystore: make add_key_origin "API-user-friendly"

Power-users that know what they are doing can use this method
to populate key origin information for keystore (bip32 root fingerprint
and derivation path prefix).
Try to make method hard to misuse.

Qt console can now be used as e.g.:
```
wallet.get_keystores()[2].add_key_origin(derivation_prefix="m/48h/1h/0h/2h", root_fingerprint="deadbeef")
```

related #5715
related #5955
related #5969

electrum/keystore.py

@@ -41,7 +41,7 @@ from .ecc import string_to_number
 from .crypto import (pw_decode, pw_encode, sha256, sha256d, PW_HASH_VERSION_LATEST,
                      SUPPORTED_PW_HASH_VERSIONS, UnsupportedPasswordHashVersion, hash_160)
 from .util import (InvalidPassword, WalletFileException,
-                   BitcoinException, bh2u, bfh, inv_dict)
+                   BitcoinException, bh2u, bfh, inv_dict, is_hex_str)
 from .mnemonic import Mnemonic, load_wordlist, seed_type, is_seed
 from .plugin import run_hook
 from .logging import Logger
@@ -463,10 +463,23 @@ class Xpub(MasterPublicKeyMixin):
         self.add_key_origin(derivation_prefix=derivation_prefix,
                             root_fingerprint=root_node.calc_fingerprint_of_this_node().hex().lower())
 
-    def add_key_origin(self, *, derivation_prefix: Optional[str], root_fingerprint: Optional[str]):
+    def add_key_origin(self, *, derivation_prefix: str = None, root_fingerprint: str = None) -> None:
         assert self.xpub
-        self._root_fingerprint = root_fingerprint
+        if not (root_fingerprint is None or (is_hex_str(root_fingerprint) and len(root_fingerprint) == 8)):
-        self._derivation_prefix = normalize_bip32_derivation(derivation_prefix)
+            raise Exception("root fp must be 8 hex characters")
+        derivation_prefix = normalize_bip32_derivation(derivation_prefix)
+        calc_root_fp, calc_der_prefix = bip32.root_fp_and_der_prefix_from_xkey(self.xpub)
+        if (calc_root_fp is not None and root_fingerprint is not None
+                and calc_root_fp != root_fingerprint):
+            raise Exception("provided root fp inconsistent with xpub")
+        if (calc_der_prefix is not None and derivation_prefix is not None
+                and calc_der_prefix != derivation_prefix):
+            raise Exception("provided der prefix inconsistent with xpub")
+        if root_fingerprint is not None:
+            self._root_fingerprint = root_fingerprint
+        if derivation_prefix is not None:
+            self._derivation_prefix = derivation_prefix
+        self.is_requesting_to_be_rewritten_to_wallet_file = True
 
     @lru_cache(maxsize=None)
     def derive_pubkey(self, for_change: int, n: int) -> bytes: