Make unit_type_data::build_unit_type() public, and update various parts of
the code to call it (instead of using unit_type_data::find() to "find" a
known unit_type).

There was one call in the editor's code where I am unsure if there are supposed
to be side effects other than building the unit_type. So I left the code as-is
and just added a comment for someone else to evaluate later.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56009 75b3775d-3801-0410-9e12-e2e43121521d