Implemented import UBZ strokes from CFF (means strokes who was exported from UBZ to CFF and imported back).
Compatible with groups.

UBDownloadManaget was adapted to use UBAsyncLocalFileDpownloader and QNetrowrkRequest bouth.

Now, only one instance per scene is allowed in calling UBGraphicsCache instance() static function.

This reverts commit 90c03c86.

Refactored downloader for local files.

User is aksed for a login+password couple if none of these have been filled in preferences.
He can allow Sankoré to store it in checking "Remember credentials".

Items duplication adapted to new ideology of adding media items on the board.