mate-user-share: update to 1.21.0

Change log:

  * Translations update
  * caja-share-bar: avoid deprecated 'g_type_class_add_private'
  * drop obsolete configure option from distcheck
  * Use make functions for HELP_LINGUAS
  * adding help to transifex config
  * disable deprecation warnings for distcheck
  * file-share-properties.ui: avoid deprecated:
  * update transifex config with branch specific resoures

(gutteridge)