task: massively simplified task system

Made various changes and fixes to the task system:

- Tasks are built around QtConcurrent rather than QThread. Reduces
  code complexity significantly.
- Task error reporting is now streamlined in both TaskManager and
  TaskDialog.
- Moved ProjectImport/Save/LoadManager to the app/task folder

app/task/proxy/proxy.h

@@ -31,8 +31,8 @@ class ProxyTask : public Task
 public:
   ProxyTask(VideoStreamPtr stream, int divider);
 
-protected:
-  virtual void Action() override;
+public slots:
+  virtual bool Run() override;
 
 private:
   VideoStreamPtr stream_;