Will list: context menu actions and will-executor bulk tools

PreviewList: right-click menu with sign / details / broadcast / check /
copy id / copy / merge from txn / delete, with multi-selection support and
availability rules (broadcast force-repushes, delete restricted to invalid
or unsigned txs, sign skips non-valid txs).

WillExecutorWidget: Export, Ping All and Select All become dropdown buttons
(export all/selected/valid, ping all/selected, select all/only valid,
deselect all/only invalid).

WillExecutorListWidget: single-cell Copy action.

BalWindow: txids-scoped sign/broadcast plumbing and merge_single_transaction.

bal/gui/qt/common.py

@@ -44,6 +44,7 @@ from electrum.gui.qt.util import (
     TaskThread,
     WindowModalDialog,
     char_width_in_lineedit,
+    getOpenFileName,
     getSaveFileName,
     import_meta_gui,
     read_QIcon_from_bytes,
@@ -78,6 +79,7 @@ from PyQt6.QtGui import QColor, QPainter, QPalette, QStandardItem, QStandardItem
 from PyQt6.QtWidgets import (
     QAbstractItemView,
     QAbstractSpinBox,
+    QApplication,
     QCheckBox,
     QComboBox,
     QDateTimeEdit,
@@ -97,6 +99,7 @@ from PyQt6.QtWidgets import (
     QStyle,
     QStyleOptionFrame,
     QTextEdit,
+    QToolButton,
     QVBoxLayout,
     QWidget,
 )