Add support for QDir mkpath when the user has DENIED_PERMISSIONS set on a directory in the middle of the path

If a directory is set up so that (for example) a user has their documents path set as 

\\\\SOMESHARE\\common_root\\user\\documents

QDir mkpath will fail to create sub directories in the documents folder if the permissions are setup so that the user is denied all permissions on (for example) \\common_root
, but given modify permissions on their user folder.

The solution would to try building the path without iterating from the drive root up, as in this example, common_root will be seen as not existing, and it cannot be created. In contrast, the following call would work

QDir("\\\\SOMESHARE\\common_root\\user\\documents
).mkdir("test");

For now the user can implement this himself but it should be a supported use case for QDir::mkpath.