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.

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

This removes a few instances of deleting a scene twice, or accessing
elements of a scene after they've been deleted.

Previously, the application would crash upon exiting if the scene was
empty but had been modified (e.g if an object was placed on the board
then deleted, then the application closed)

This prevents crashes that may occur when deleting multiple pages from a document.

(Reading is still done synchronously, for now at least)

Threaded the storage of the scene. Asynchronous reading of the previous and next scene to be stored on cache.

persistence of not required media on disk. Second step updated the cleanSource methods in classes that handle files on disk

Document navigator: "Add pages from file" isn't work with old UBZ

Show Desktop: "Erase all" button isn't work

Document explorer(navigator): Imported ubz files are always imported into "Untitled Documents" folder

Fix for losing content mentioned in the ticket 228. The problem was actually in page deletion algorithm which has been flooding cache with invalid data, and that data was a object to return when loadDocumentScene was called.