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/project/save/save.cpp

@@ -0,0 +1,63 @@
+/***
+
+  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/>.
+
+***/
+
+#include "save.h"
+
+#include <QFile>
+#include <QXmlStreamWriter>
+
+OLIVE_NAMESPACE_ENTER
+
+ProjectSaveTask::ProjectSaveTask(ProjectPtr project) :
+  project_(project)
+{
+  SetTitle(tr("Saving '%1'").arg(project->filename()));
+}
+
+bool ProjectSaveTask::Run()
+{
+  QFile project_file(project_->filename());
+
+  if (project_file.open(QFile::WriteOnly | QFile::Text)) {
+    QXmlStreamWriter writer(&project_file);
+    writer.setAutoFormatting(true);
+
+    writer.writeStartDocument();
+
+    writer.writeStartElement("olive");
+
+    writer.writeTextElement("version", "0.2.0");
+
+    project_->Save(&writer);
+
+    writer.writeEndElement(); // olive
+
+    writer.writeEndDocument();
+
+    project_file.close();
+
+    return true;
+  } else {
+    SetError(tr("Failed to open file \"%1\" for writing.").arg(project_->filename()));
+    return false;
+  }
+}
+
+OLIVE_NAMESPACE_EXIT