qml: rbf/cancel: fix type error

self.oldfeeRate is initialized as int in `QETxRbfFeeBumper` and
`QETxCanceller`. However QML expects it to be a string. Initializing
it as string fixes the exception. Previously this didn't happen as
`Abstract_Wallet.add_info_from_wallet_and_network()` would never
return False, so the `get_tx()` method would immediately overwrite
the self.oldfeeRate variable with a string.

```
 20.95 | D | gui.qml.qetxfinalizer | TxMonMixin.__init__
 20.95 | E | gui.qml.qeapp.Exception_Hook | exception caught by crash reporter
TypeError: unable to convert a Python 'int' object to a C++ 'QString' instance
```

electrum/gui/qml/qetxfinalizer.py

@@ -659,7 +659,7 @@ class QETxRbfFeeBumper(TxFeeSlider, TxMonMixin):
         super().__init__(parent)
 
         self._oldfee = QEAmount()
-        self._oldfee_rate = 0
+        self._oldfee_rate = '0'
         self._orig_tx = None
         self._rbf = True
         self._bump_method = BumpFeeStrategy.PRESERVE_PAYMENT.name
@@ -800,7 +800,7 @@ class QETxCanceller(TxFeeSlider, TxMonMixin):
         super().__init__(parent)
 
         self._oldfee = QEAmount()
-        self._oldfee_rate = 0
+        self._oldfee_rate = '0'
         self._orig_tx = None
         self._txid = ''
         self._rbf = True