Add support for storing settings in localStorage

This commit adds an option to store jscut settings in the browser local
storage.
Fix bugs in local storage

* data-bind syntax error caused the select in
  load-local-storage-settings-modal to not set saveSettingsFilename.

* Binding saveSettingsFilename to the select prevented the user from
  entering unique values in the save dialog; the select's data validation
  reset it to an already existing value.