Revert "due to suspected instability with persistance Worker (randomly not...

Revert "due to suspected instability with persistance Worker (randomly not persisted scenes, very hard to reproduce), switched to 'immediateSave mode' (at least for now)"

This reverts commit ae1a2a53.

src/core/UBPersistenceManager.h

@@ -77,7 +77,7 @@ class UBPersistenceManager : public QObject
         virtual void duplicateDocumentScene(UBDocumentProxy* pDocumentProxy, int index);
 
         virtual void persistDocumentScene(UBDocumentProxy* pDocumentProxy,
-                UBGraphicsScene* pScene, const int pSceneIndex, bool isAnAutomaticBackup = false, bool forceImmediateSaving = true);
+                UBGraphicsScene* pScene, const int pSceneIndex, bool isAnAutomaticBackup = false, bool forceImmediateSaving = false);
 
         virtual UBGraphicsScene* createDocumentSceneAt(UBDocumentProxy* pDocumentProxy, int index, bool useUndoRedoStack = true);