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/render/render.h

@@ -0,0 +1,49 @@
+/***
+
+  Olive - Non-Linear Video Editor
+  Copyright (C) 2019 Olive Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#ifndef RENDERTASK_H
+#define RENDERTASK_H
+
+#include "node/output/viewer/viewer.h"
+#include "task/task.h"
+
+OLIVE_NAMESPACE_ENTER
+
+class RenderTask : public Task
+{
+public:
+  RenderTask(ViewerOutput* viewer);
+
+protected:
+  void Render(TimeRangeList range_to_cache, int divider = 1);
+
+  ViewerOutput* viewer() const
+  {
+    return viewer_;
+  }
+
+private:
+  ViewerOutput* viewer_;
+
+};
+
+OLIVE_NAMESPACE_EXIT
+
+#endif // RENDERTASK_H