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/window/mainwindow/mainmenu.cpp

@@ -29,7 +29,6 @@
 #include "dialog/actionsearch/actionsearch.h"
 #include "dialog/task/task.h"
 #include "panel/panelmanager.h"
-#include "task/cache/cache.h"
 #include "tool/tool.h"
 #include "ui/style/style.h"
 #include "undo/undostack.h"
@@ -613,21 +612,12 @@ void MainMenu::OpenRecentItemTriggered()
 
 void MainMenu::SequenceCacheTriggered()
 {
-  TimeBasedPanel* p = PanelManager::instance()->MostRecentlyFocused<TimeBasedPanel>();
-
-  if (p && p->GetConnectedViewer()) {
-    // FIXME: Hardcoded divider...
-    // FIXME: Consider preventing caching the footage viewer
-    CacheTask* task = new CacheTask(p->GetConnectedViewer(), 2, false);
-
-    TaskDialog* dialog = new TaskDialog(task, tr("Caching Sequence"), parentWidget());
-    dialog->open();
-  }
+  Core::instance()->CacheActiveSequence(false);
 }
 
 void MainMenu::SequenceCacheInOutTriggered()
 {
-  qDebug() << "STUB";
+  Core::instance()->CacheActiveSequence(true);
 }
 
 void MainMenu::Retranslate()