client: stretches: fixed highlighting of selected stretch The vectorloader for each layer can now take a callback to postprocess the returned features. For the stretches layer the selectedStretch (which is now stored in the vuex store) now gets a 'highlighted' flag. The layer respects that flag to alter the styles. A watcher on selectedStretch resets the styles when selectedStretch changes and sets the highlighted flag for the new selectedStretch, if present, otherwise the vectorloader does this job.