qml: fix dialog not closing if dialog is shown/hidden more than once

most dialogs are destroyed when closed, therefore not hitting this bug,
but e.g. ChannelOpenProgressDialog is reused.

electrum/gui/qml/components/controls/ElDialog.qml

@@ -49,6 +49,7 @@ Dialog {
         if (opened) {
             app.activeDialogs.push(abstractdialog)
             _wasOpened = true
+            _result = false
         } else {
             if (!_wasOpened)
                 return