various small code cleanups and improvements

Largely refactoring work to make the code somewhat nicer to work with.

app/task/taskmanager.h

@@ -70,6 +70,8 @@ public:
    */
   TaskManager& operator=(TaskManager&& other) = delete;
 
+  static TaskManager* instance();
+
   /**
    * @brief Add a new Task
    *
@@ -165,6 +167,11 @@ private:
    */
   int maximum_task_count_;
 
+  /**
+   * @brief TaskManager singleton instance
+   */
+  static TaskManager instance_;
+
 private slots:
   /**
    * @brief Callback when a Task's status changes
@@ -180,8 +187,4 @@ private slots:
 
 };
 
-namespace olive {
-extern TaskManager task_manager;
-}
-
 #endif // TASKMANAGER_H