DnD/X11: generalize atoms descriptions
DnD/X11: generalize atoms descriptions

This step is needed to simplify types management for Wayland by using
the same data structures.

rvandegrift/elementary,rvandegrift/elementary,FlorentRevest/Elementary,rvandegrift/elementary,tasn/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,tasn/elementary,rvandegrift/elementary,tasn/elementary,tasn/elementary,tasn/elementary