wallet: add get_label_for_address, and make get_label private

fixes https://github.com/spesmilo/electrum/issues/7919

In the past, when creating payment requests, we keyed them by on-chain address,
and set/saved the msg of the request as label for the address.
Many places in the code were calling wallet.get_label(addr) with the expectation that
relevant payment requests are found and their message/description (if any) is considered.

wallet.get_label(key) is now made private, and instead the explicit non-polymorphic
wallet.get_label_for_{address,rhash,txid} alternatives should be used.

electrum/gui/qml/qewallet.py

@@ -2,7 +2,7 @@ import asyncio
 import queue
 import threading
 import time
-from typing import Optional
+from typing import Optional, TYPE_CHECKING
 
 from PyQt5.QtCore import pyqtProperty, pyqtSignal, pyqtSlot, QObject, QTimer
 
@@ -22,6 +22,10 @@ from .qetransactionlistmodel import QETransactionListModel
 from .qetypes import QEAmount
 from .util import QtEventListener, qt_event_listener
 
+if TYPE_CHECKING:
+    from electrum.wallet import Abstract_Wallet
+
+
 class QEWallet(AuthMixin, QObject, QtEventListener):
     __instances = []
 
@@ -62,7 +66,7 @@ class QEWallet(AuthMixin, QObject, QtEventListener):
     _synchronizing = False
     _synchronizing_progress = ''
 
-    def __init__(self, wallet, parent=None):
+    def __init__(self, wallet: 'Abstract_Wallet', parent=None):
         super().__init__(parent)
         self.wallet = wallet