Fix Bug 438084 - Preserve Nextcloud path when returning dav data
https://bugs.kde.org/show_bug.cgi?id=438084 describes how the Nextcloud path is stripped from the URL when passed back to kio-webdav
This merge request fixes this bug by building up the path using the original path as a starting point.
This merge request also changes how the login credentials returned from the Nextcloud Login Flow are processed, preferring QUrlQuery over string manipulation.
Tested on Plasma 5.22.2, Framework 5.83.0, Qt 5.15.3, Kernel 5.8