libdoomsday|Uri: Use Path::withSeparators()

The previous implementation would swap separator characters decoded
from the percent encoding, too.
Merge branch 'master' into defs-v2
Merge branch 'defs-v2' of ssh://github.com/skyjake/Doomsday-Engine into defs-v2
Merge branch 'defs-v2' of ssh://github.com/skyjake/Doomsday-Engine into defs-v2