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/export/exportparams.h

@@ -0,0 +1,73 @@
+/***
+
+  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 EXPORTPARAMS_H
+#define EXPORTPARAMS_H
+
+#include <QMatrix4x4>
+
+#include "codec/encoder.h"
+#include "node/output/viewer/viewer.h"
+#include "render/colortransform.h"
+
+OLIVE_NAMESPACE_ENTER
+
+class ExportParams : public EncodingParams {
+public:
+  enum VideoScalingMethod {
+    kFit,
+    kStretch,
+    kCrop
+  };
+
+  ExportParams();
+
+  const QString& encoder() const;
+  void set_encoder(const QString& id);
+
+  bool has_custom_range() const;
+  const TimeRange& custom_range() const;
+  void set_custom_range(const TimeRange& custom_range);
+
+  const VideoScalingMethod& video_scaling_method() const;
+  void set_video_scaling_method(const VideoScalingMethod& video_scaling_method);
+
+  const ColorTransform& color_transform() const;
+  void set_color_transform(const ColorTransform& color_transform);
+
+  static QMatrix4x4 GenerateMatrix(ExportParams::VideoScalingMethod method,
+                                   int source_width, int source_height,
+                                   int dest_width, int dest_height);
+
+private:
+  QString encoder_id_;
+
+  VideoScalingMethod video_scaling_method_;
+
+  bool has_custom_range_;
+  TimeRange custom_range_;
+
+  ColorTransform color_transform_;
+
+};
+
+OLIVE_NAMESPACE_EXIT
+
+#endif // EXPORTPARAMS_H