redraw_unit takes display & as an argument
split off animation fcs from unit to unit_animation_component

This is a strict refactor, all we do is move the functions and
variables used just for animations to "unit_animation_component",
and include the necessary headers appropriate.

With a bit more work we can probably remove the graphics related
headers from unit.hpp
Merge branch 'master' of git://github.com/wesnoth/wesnoth