Properly handle denormalized href

Properly handle denormalized href

In case of denormalized paths in the dav href (presence of . or .. in
the path) simple string startsWith comparison wasn't enough to know if
said href ended up in the right namespace. That's why we're now using
QUrl (pretending local file since we don't have a full URL in the href)
to normalize the path before comparison.

This could happen with broken proxies for instance where we would
wrongly validate the dav information resulting in potentially surprising
syncing and name collisions.

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>