Add new AI aspect leader_ignores_keep.  If set, AI leaders do not move to
the closest keep at the beginning of the turn.  Instead, they participate 
in the move_to_targets candidate action (and all other CAs in which they
already participated anyway, of course).  The default value is 'no', which
leaves the default behavior unchanged.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56101 75b3775d-3801-0410-9e12-e2e43121521d