[SMB] Fix stat and unlink

Also:
- fixed delete action to show error message as notification
- added unit tests for getId() and constructUrl() for SMB module
- added storage unit test for unlink operation
- added workaround to detect successful unlink operation for SMB
- fixes #5778
- fixes #5866

Please review @icewind1991 @DeepDiver1975 @karlitschek

Note: I've put the fixes together because they all rely on stat() to work at all.