cache: reimplemented auto-caching

Implemented a smarter auto-cache that's context sensitive. Caching will
automatically pause when playback begins or values are being changed and resume
when inactive.

app/task/taskmanager.h

@@ -63,6 +63,8 @@ public:
 
   Task* GetFirstTask() const;
 
+  void CancelTaskAndWait(Task* t);
+
 public slots:
   /**
    * @brief Add a new Task
@@ -81,6 +83,8 @@ public slots:
    */
   void AddTask(Task *t);
 
+  void CancelTask(Task* t);
+
 signals:
   /**
    * @brief Signal emitted when a Task is added by AddTask()