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/load/CMakeLists.txt

@@ -0,0 +1,22 @@
+# 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/>.
+
+set(OLIVE_SOURCES
+  ${OLIVE_SOURCES}
+  task/project/load/load.h
+  task/project/load/load.cpp
+  PARENT_SCOPE
+)

app/task/project/load/load.cpp

@@ -0,0 +1,86 @@
+/***
+
+  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 "load.h"
+
+#include <QApplication>
+#include <QFile>
+#include <QXmlStreamReader>
+
+#include "common/xmlutils.h"
+
+OLIVE_NAMESPACE_ENTER
+
+ProjectLoadTask::ProjectLoadTask(const QString &filename) :
+  filename_(filename)
+{
+  SetTitle(tr("Loading '%1'").arg(filename));
+}
+
+bool ProjectLoadTask::Run()
+{
+  QFile project_file(filename_);
+
+  if (project_file.open(QFile::ReadOnly | QFile::Text)) {
+    QXmlStreamReader reader(&project_file);
+
+    while (XMLReadNextStartElement(&reader)) {
+      if (reader.name() == QStringLiteral("olive")) {
+        while(XMLReadNextStartElement(&reader)) {
+          if (reader.name() == QStringLiteral("version")) {
+            qDebug() << "Project version:" << reader.readElementText();
+          } else if (reader.name() == QStringLiteral("project")) {
+            ProjectPtr project = std::make_shared<Project>();
+
+            project->set_filename(filename_);
+
+            project->Load(&reader, &IsCancelled());
+
+            // Ensure project is in main thread
+            moveToThread(qApp->thread());
+
+            if (!IsCancelled()) {
+              projects_.append(project);
+            }
+          } else {
+            reader.skipCurrentElement();
+          }
+        }
+      } else {
+        reader.skipCurrentElement();
+      }
+    }
+
+    project_file.close();
+
+    if (reader.hasError()) {
+      SetError(reader.errorString());
+      return false;
+    } else {
+      return true;
+    }
+
+  } else {
+    SetError(tr("Failed to read file \"%1\" for reading.").arg(filename_));
+    return false;
+  }
+}
+
+OLIVE_NAMESPACE_EXIT

app/task/project/load/load.h

@@ -0,0 +1,52 @@
+/***
+
+  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 PROJECTLOADMANAGER_H
+#define PROJECTLOADMANAGER_H
+
+#include "project/project.h"
+#include "task/task.h"
+
+OLIVE_NAMESPACE_ENTER
+
+class ProjectLoadTask : public Task
+{
+  Q_OBJECT
+public:
+  ProjectLoadTask(const QString& filename);
+
+  const QList<ProjectPtr>& GetLoadedProjects()
+  {
+    return projects_;
+  }
+
+public slots:
+  virtual bool Run() override;
+
+private:
+  QList<ProjectPtr> projects_;
+
+  QString filename_;
+
+};
+
+OLIVE_NAMESPACE_EXIT
+
+#endif // PROJECTLOADMANAGER_H