Add a dialog explaining what to do next when tapping "Verify this device"

The menu that has this action is now more discoverable, and this menu item is
plagued with a bad UX - when you tap on it nothing obvious happens! To people
not familiar with device verification they will think this is a bug, but in fact
all they need to do is open another verified session on another device.

So now there's a dialog explaining that the next step is to do just that. This
dialog also closes once the verification session starts, but the user has the
option to close it in the meantime.

src/qml/AccountMenu.qml

@@ -87,7 +87,18 @@ KirigamiComponents.ConvergentContextMenu {
     QQC2.Action {
         text: i18nc("@action:inmenu", "Verify This Device")
         icon.name: "security-low"
-        onTriggered: root.connection.startSelfVerification()
+        onTriggered: {
+            root.connection.startSelfVerification();
+            const dialog = Qt.createComponent("org.kde.kirigami", "PromptDialog").createObject(QQC2.Overlay.overlay, {
+                title: i18nc("@title", "Verification Request Sent"),
+                subtitle: i18nc("@info:label", "To proceed, accept the verification request on another device."),
+                standardButtons: Kirigami.Dialog.Ok
+            })
+            dialog.open();
+            root.connection.onNewKeyVerificationSession.connect(() => {
+                dialog.close();
+            });
+        }
         enabled: Controller.csSupported
     }