qml: show channel backup and explanatory message before local force close, and let user confirm before doing the close operation

also show message dialog after close succeeded instead of just closing the channel close dialog

electrum/gui/qml/qechanneldetails.py

@@ -13,6 +13,7 @@ from .qewallet import QEWallet
 from .qetypes import QEAmount
 from .util import QtEventListener, qt_event_listener, event_listener
 
+
 class QEChannelDetails(QObject, QtEventListener):
     _logger = get_logger(__name__)
 
@@ -171,13 +172,26 @@ class QEChannelDetails(QObject, QtEventListener):
         return self._channel.can_be_deleted()
 
     @pyqtProperty(str, notify=channelChanged)
-    def messageForceClose(self, notify=channelChanged):
+    def messageForceClose(self):
         return messages.MSG_REQUEST_FORCE_CLOSE.strip()
 
+    @pyqtProperty(str, notify=channelChanged)
+    def messageForceCloseBackup(self):
+        return ' '.join([
+            _('If you force-close this channel, the funds you have in it will not be available for {} blocks.').format(self.toSelfDelay),
+            _('During that time, funds will not be recoverable from your seed, and may be lost if you lose your device.'),
+            _('To prevent that, please save this channel backup.'),
+            _('It may be imported in another wallet with the same seed.')
+        ])
+
     @pyqtProperty(bool, notify=channelChanged)
     def isBackup(self):
         return self._channel.is_backup()
 
+    @pyqtProperty(int, notify=channelChanged)
+    def toSelfDelay(self):
+        return self._channel.config[REMOTE].to_self_delay
+
     @pyqtSlot()
     def freezeForSending(self):
         lnworker = self._channel.lnworker