feat: scan existing systemd unit

We should scan existing systemd unit when we start application manager.
As application manager crash and restarted later,
we can get the launched application instances back.

src/dbus/applicationmanager1service.h

@@ -53,6 +53,11 @@ private:
     std::unique_ptr<Identifier> m_identifier;
     QScopedPointer<JobManager1Service> m_jobManager{nullptr};
     QMap<QDBusObjectPath, QSharedPointer<ApplicationService>> m_applicationList;
+
+    void scanApplications() noexcept;
+    void scanInstances() noexcept;
+    void addInstanceToApplication(const QString &unitName, const QDBusObjectPath &systemdUnitPath);
+    void removeInstanceFromApplication(const QString &unitName, const QDBusObjectPath &systemdUnitPath);
 };
 
 #endif