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/commands.py

@@ -52,7 +52,7 @@ from .transaction import (Transaction, multisig_script, TxOutput, PartialTransac
 from . import transaction
 from .invoices import PR_PAID, PR_UNPAID, PR_UNKNOWN, PR_EXPIRED
 from .synchronizer import Notifier
-from .wallet import Abstract_Wallet, create_new_wallet, restore_wallet_from_text, Deterministic_Wallet
+from .wallet import Abstract_Wallet, create_new_wallet, restore_wallet_from_text, Deterministic_Wallet, BumpFeeStrategy
 from .address_synchronizer import TX_HEIGHT_LOCAL
 from .mnemonic import Mnemonic
 from .lnutil import SENT, RECEIVED
@@ -819,7 +819,7 @@ class Commands:
             tx=tx,
             txid=tx.txid(),
             coins=coins,
-            decrease_payment=decrease_payment,
+            strategy=BumpFeeStrategy.DECREASE_PAYMENT if decrease_payment else BumpFeeStrategy.PRESERVE_PAYMENT,
             new_fee_rate=new_fee_rate)
         if not unsigned:
             wallet.sign_transaction(new_tx, password)