Improve the notification setting description

The current text has invited a lot of confusion around how notifications
work in NeoChat, because it mentions "push notifications". Some users
take it to mean that somehow the notifications appear in the background,
but that's only supported if built with KUnifiedPush.

To make it super clear, let's change the description dynamically based
on whether:
1. NeoChat is built with KUnifiedPush support.
2. We were able to connect with the KUnifiedPush daemon and your server
has a push gateway.

src/neochatconnection.h

@@ -90,6 +90,16 @@ class NeoChatConnection : public Quotient::Connection
      */
     Q_PROPERTY(bool canEraseData READ canEraseData NOTIFY canEraseDataChanged)
 
+    /**
+     * @brief Whether this build of NeoChat supports push notifications via KUnifiedPush.
+     */
+    Q_PROPERTY(bool pushNotificationsAvailable READ pushNotificationsAvailable CONSTANT)
+
+    /**
+     * @brief Whether we successfully set up push notifications with the server.
+     */
+    Q_PROPERTY(bool enablePushNotifications READ enablePushNotifications NOTIFY enablePushNotificationsChanged)
+
 public:
     /**
      * @brief Defines the status after an attempt to change the password on an account.
@@ -178,6 +188,9 @@ public:
     // the reference could be destroyed before the task is finished
     QCoro::Task<void> setupPushNotifications(QString endpoint);
 
+    bool pushNotificationsAvailable() const;
+    bool enablePushNotifications() const;
+
     bool isOnline() const;
 
     LinkPreviewer *previewerForLink(const QUrl &link);
@@ -196,6 +209,7 @@ Q_SIGNALS:
     void badgeNotificationCountChanged(NeoChatConnection *connection, int count);
     void canCheckMutualRoomsChanged();
     void canEraseDataChanged();
+    void enablePushNotificationsChanged();
 
     /**
      * @brief Request a message be shown to the user of the given type.
@@ -221,4 +235,5 @@ private:
 
     bool m_canCheckMutualRooms = false;
     bool m_canEraseData = false;
+    bool m_pushNotificationsEnabled = false;
 };