fixes up commit 82c6b98907d9709aef0d23a3846c1e75ac48e1d5
Use smart "handles" for halos which have been added to a halo
manager. The handles remember what manager they came from, and
delete themselves automatically on destruction.
This wasn't an issue when haloes were basically managed by a
C library, but if we want to get rid of the static singleton
system, the handles need to be smarter than just int's.
This allows us to eliminate two headers and move code out of
play_controller, and keep related code together. The cache is
now a static variable in the only function that calls it.
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
