udisks2: add udisks2 service

Udisks2 is a Dbus service started by systemd on demand.
Xfce: use udisks2 instead of udisks

Enabling udisks2 allows gvfs (which should be built with udisks support) to
perform mount/unmount operations on removable drives. It affects Thunar and
probably other Gtk-based file managers.

Gvfs doesn't supports earlier versions of udisks.
Merge pull request #132 from ierton/udisks2-v2

add udisks2 for Xfce