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

Have a problem - should take care about saving original src path.

Still have some mesh with undo stack and with widget content.

1st we remove original widget, then we disable undo stack, create a copy of widget and make it a tool. Then undo stack enables back. That widget has tool as parent, so we not need to manage him by scene.

Import PDF: progress bar is not visible

Now coping creates a full copy of already created item. Copy has it's own content (like video, audio or folder) and stores original item properties (like locked, pos, size, freezed e.t.c.).

The old code has to be removed and remaining bugs fixed.

Fixed pages shuffling.
Fixed refreshing thumbnail of new added page.
Some code refactoring and optimizations.

Frame buttons

Document explorer(navigator): pages of a document can not be moved using D'n'D