KUrlNavigator: always use "desktop:/" not "desktop:"

Navigating to "desktop:" in e.g. dolphin, then creating a new folder would lead to creating "/home/user/DesktopNew Folder", that's because the "path" segment in the url is empty. The fix is in two parts, one here by ensuring we always go to "desktop:/", even if the user typed only "desktop:". Do this for all KProtocols that have class ":local"; something like "file:somepath" would treat somepath as a relative path (in dolphin, relative to the current dir), we lose that "functionality", but I think it's too obscure, so losing it is not a big deal.

The other part is in kio_desktop in plasma-workspace.

Add unit test.