Add Windows-specific notation for WebDAV to QUrl.

Convert a Windows-specific WebDAV specification
"//host@SSL/path" into URL's with scheme set to
"webdavs" and back to local file (Windows only).

Task-number: QTBUG-42346
Change-Id: I12663243848ea7b2d3f208743e837e9de14a93eb
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>