wallet.bump_fee: the glorious return of BumpFeeStrategy :D

gui/qt/rbf_dialog.py (old) lines 57-64 were implementing logic that should not be part of GUI code.
Case in point, gui/qml/qetxfinalizer.py (old) lines 511-513 duplicated half of that logic but not the other half.
That logic is now moved to wallet.get_bumpfee_strategies_for_tx().

More context: a user on irc got confused when using the qml gui. They were sending "max" and wanted to bump_fee.
The qml gui selected the "preserve_payment" strategy by default, using which there was no solution, and the user
did not notice that the strategy can be altered (via the "method" dropdown). The qt gui had logic to select
"decrease_payment" by default in such a case (which does find a solution to bump) but this logic was not
duplicated in the qml gui.
Instead of duplicating the logic, this commit moves it to shared lib code.

electrum/gui/qml/components/RbfBumpFeeDialog.qml

@@ -62,15 +62,11 @@ ElDialog {
 
                     ElComboBox {
                         id: bumpMethodComboBox
-                        enabled: rbffeebumper.canChangeBumpMethod
 
                         textRole: 'text'
                         valueRole: 'value'
 
-                        model: [
-                            { text: qsTr('Preserve payment'), value: 'preserve_payment' },
-                            { text: qsTr('Decrease payment'), value: 'decrease_payment' }
-                        ]
+                        model: rbffeebumper.bumpMethodsAvailable
                         onCurrentValueChanged: {
                             if (activeFocus)
                                 rbffeebumper.bumpMethod = currentValue

electrum/gui/qml/qetxfinalizer.py

@@ -8,7 +8,7 @@ from electrum.logging import get_logger
 from electrum.i18n import _
 from electrum.transaction import PartialTxOutput, PartialTransaction, Transaction
 from electrum.util import NotEnoughFunds, profiler
-from electrum.wallet import CannotBumpFee, CannotDoubleSpendTx, CannotCPFP
+from electrum.wallet import CannotBumpFee, CannotDoubleSpendTx, CannotCPFP, BumpFeeStrategy
 from electrum.plugin import run_hook
 
 from .qewallet import QEWallet
@@ -460,13 +460,8 @@ class QETxRbfFeeBumper(TxFeeSlider, TxMonMixin):
         self._oldfee_rate = 0
         self._orig_tx = None
         self._rbf = True
-        self._bump_method = 'preserve_payment'
-        self._can_change_bump_method = True
-
-    canChangeBumpMethodChanged = pyqtSignal()
-    @pyqtProperty(bool, notify=canChangeBumpMethodChanged)
-    def canChangeBumpMethod(self):
-        return self._can_change_bump_method
+        self._bump_method = BumpFeeStrategy.PRESERVE_PAYMENT.name
+        self._bump_methods_available = []
 
     oldfeeChanged = pyqtSignal()
     @pyqtProperty(QEAmount, notify=oldfeeChanged)
@@ -496,21 +491,26 @@ class QETxRbfFeeBumper(TxFeeSlider, TxMonMixin):
         return self._bump_method
 
     @bumpMethod.setter
-    def bumpMethod(self, bumpmethod):
-        assert self._can_change_bump_method
+    def bumpMethod(self, bumpmethod: str) -> None:
         if self._bump_method != bumpmethod:
             self._bump_method = bumpmethod
             self.bumpMethodChanged.emit()
             self.update()
 
+    bumpMethodsAvailableChanged = pyqtSignal()
+    @pyqtProperty('QVariantList', notify=bumpMethodsAvailableChanged)
+    def bumpMethodsAvailable(self):
+        return self._bump_methods_available
+
     def get_tx(self):
         assert self._txid
         self._orig_tx = self._wallet.wallet.db.get_transaction(self._txid)
         assert self._orig_tx
 
-        if self._wallet.wallet.get_swap_by_funding_tx(self._orig_tx):
-            self._can_change_bump_method = False
-            self.canChangeBumpMethodChanged.emit()
+        strategies, def_strat_idx = self._wallet.wallet.get_bumpfee_strategies_for_tx(tx=self._orig_tx, txid=self._txid)
+        self._bump_methods_available = [{'value': strat.name, 'text': strat.text()} for strat in strategies]
+        self.bumpMethodsAvailableChanged.emit()
+        self.bumpMethod = strategies[def_strat_idx].name
 
         if not isinstance(self._orig_tx, PartialTransaction):
             self._orig_tx = PartialTransaction.from_tx(self._orig_tx)
@@ -547,7 +547,7 @@ class QETxRbfFeeBumper(TxFeeSlider, TxMonMixin):
                 tx=self._orig_tx,
                 txid=self._txid,
                 new_fee_rate=new_fee_rate,
-                decrease_payment=self._bump_method=='decrease_payment'
+                strategy=BumpFeeStrategy[self._bump_method],
             )
         except CannotBumpFee as e:
             self._valid = False