xfce4-whiskermenu-plugin: update to 2.8.0

AccountsService and some Wayland-specific functionality are not
enabled for pkgsrc at present. AccountsService is too out of date and
bringing it fully forward seems non-trivial (though we could more
easily roll it forward to the absolute minimum this package expects).

Change log:

2.8.0
=====
- Fix menu toggling in Wayland.
- Fix menu position in Wayland.
- Add support for popping up specific menu instances. (Issue #95)
- Add showing menu at center of screen. (Issue #109)
- Replace options for positioning menu layout.
- Replace resize widgets with options in settings dialog.
- Replace command for switching users.
- Replace popup script with executable.
- Require Xfce 4.14.
- Make linking against AccountsService explicit.
- Stop including favorites in recent by default.
- Store settings in Xfconf.
- Translation updates: Bulgarian, Chinese (Taiwan), Danish, Dutch, German,
  Greek, Spanish, Basque, Finnish, French, Italian, Lithuanian, Persian,
  Portuguese, Russian, Slovak, Swedish, Turkish, Ukrainian.

(gutteridge)