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/widget/taskview/taskview.cpp

@@ -46,7 +46,20 @@ TaskView::TaskView(QWidget* parent) :
 void TaskView::AddTask(Task *t)
 {
   // Create TaskViewItem (UI representation of a Task) and connect it
-  layout_->insertWidget(layout_->count()-1, new TaskViewItem(t));
+  TaskViewItem* item = new TaskViewItem(t);
+  items_.insert(t, item);
+  layout_->insertWidget(layout_->count()-1, item);
+}
+
+void TaskView::TaskFailed(Task *t)
+{
+  items_.value(t)->Failed();
+}
+
+void TaskView::RemoveTask(Task *t)
+{
+  items_.value(t)->deleteLater();
+  items_.remove(t);
 }
 
 OLIVE_NAMESPACE_EXIT

app/widget/taskview/taskview.h

@@ -50,9 +50,17 @@ public slots:
    */
   void AddTask(Task* t);
 
+  void TaskFailed(Task* t);
+
+  void RemoveTask(Task* t);
+
 private:
   QWidget* central_widget_;
+
   QVBoxLayout* layout_;
+
+  QHash<Task*, TaskViewItem*> items_;
+
 };
 
 OLIVE_NAMESPACE_EXIT

app/widget/taskview/taskviewitem.cpp

@@ -61,8 +61,12 @@ TaskViewItem::TaskViewItem(Task* task, QWidget *parent) :
 
   // Connect to the task
   connect(task_, &Task::ProgressChanged, progress_bar_, &QProgressBar::setValue);
-  connect(task_, &Task::Removed, this, &TaskViewItem::deleteLater);
   connect(cancel_btn_, &QPushButton::clicked, task_, &Task::Cancel, Qt::DirectConnection);
 }
 
+void TaskViewItem::Failed()
+{
+  task_status_lbl_->setText(task_->GetError());
+}
+
 OLIVE_NAMESPACE_EXIT

app/widget/taskview/taskviewitem.h

@@ -45,6 +45,8 @@ class TaskViewItem : public QFrame
 public:
   TaskViewItem(Task *task, QWidget* parent = nullptr);
 
+  void Failed();
+
 private:
   QLabel* task_name_lbl_;
   QProgressBar* progress_bar_;