Have unit store a pointer to unit_type instead of a unit_type ID string.

In addition to speeding access to the unit_type (no need to search), this makes it
easier to see that a unit always has a valid unit_type (an exception is thrown otherwise).


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56062 75b3775d-3801-0410-9e12-e2e43121521d