4.15.3

- Update bug URLs to point to gitlab.xfce
- xfce-about: Hide empty email address tags (Fixes #12)
- Remove translator email address as per request (Fixes #11)
- Add missing translator "Hugo Carvalho" to xfce4-about (issue #18)
- Replace deprecated 'gdk_keyboard_ungrab' function (Fixes #15)
- shortcuts-grabber: Correctly ungrab keys on updating shortcuts (Fixes
  #16)
- Add basic GitLab pipeline
- Allow more granular control over menu item accelerators.
  - No automatic accelerator activation formenu items
  - Added 'gtk_menu_item_set_accel_label'
  - Added 'xfce_gtk_accel_group_connect_action_entries'
- renamed 'xfce_gtk_accel_group_append' to
  'xfce_gtk_accel_map_add_entries' and modified internals accordingly.
  No automatic add to an accel_group any more, since this may lead to
  double-activation in some cases.
- Add "xfce_gtk_accel_group_disconnect_action_entries" to be able to
  revert "xfce_gtk_accel_group_connect_action_entries"
- Add title to xfce_dialog_confirm_close_tabs
- Add new methods to libxfce4ui.symbols. Followup of
  231e57daccb5946379f413ed440629c32025f6f3
- Translation Updates:
  Albanian, Amharic, Arabic, Armenian, Armenian (Armenia), Basque,
  Belarusian, Bengali, Bulgarian, Catalan, Chinese (China), Chinese
(Hong
  Kong), Chinese (Taiwan), Croatian, Czech, Danish, Dutch, Eastern
  Armenian, English (Australia), English (United Kingdom), Estonian,
  Finnish, French, Galician, German, Greek, Hebrew, Hungarian,
Icelandic,
  Indonesian, Interlingue, Italian, Japanese, Kazakh, Korean, Latvian,
  Lithuanian, Malay, Norwegian Bokmål, Norwegian Nynorsk, Occitan (post
  1500), Polish, Portuguese, Portuguese (Brazil), Romanian, Russian,
  Serbian, Slovak, Slovenian, Spanish, Swedish, Telugu, Thai, Turkish,
  Uighur, Ukrainian, Welsh