This fixes two related issues:
1) When taking a partial screenshot of the desktop, then copying it
(Ctrl-C) and pasting it in a new document, it was not saved
2) When a page of one document was copied into another (in document
mode, by dragging the page onto another document), media files
disappeared from the new page.

When drag-n-dropping a document to the trash, it would reappear in the
"untitled documents" folder on the next restart. (This was introduced by
bd3d8e95)

Also quick code clean-up in UBDocumentController

The empty name should be avoided and the same directory name too. File name could have the same name this is as before

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