qml: confirm close electrum dialog instead of double-tap back button

electrum/gui/qml/components/main.qml

@@ -30,6 +30,8 @@ ApplicationWindow
 
     property variant activeDialogs: []
 
+    property bool _wantClose: false
+
     header: ToolBar {
         id: toolbar
 
@@ -302,23 +304,25 @@ ApplicationWindow
             stack.pop()
         } else {
             // destroy most GUI components so that we don't dump so many null reference warnings on exit
-            if (closeMsgTimer.running) {
+            if (app._wantClose) {
                 app.header.visible = false
                 mainStackView.clear()
             } else {
-                notificationPopup.show('Press Back again to exit')
-                closeMsgTimer.start()
+                var dialog = app.messageDialog.createObject(app, {
+                    text: qsTr('Close Electrum?'),
+                    yesno: true
+                })
+                dialog.yesClicked.connect(function() {
+                    dialog.close()
+                    app._wantClose = true
+                    app.close()
+                })
+                dialog.open()
                 close.accepted = false
             }
         }
     }
 
-    Timer {
-        id: closeMsgTimer
-        interval: 5000
-        repeat: false
-    }
-
     Connections {
         target: Daemon
         function onWalletRequiresPassword() {