New scheme for coloring vision costs in the help: white for "normal" (that is, same as
movement cost), yellow for "worse" (more than movement cost), and green for "better" (less
than movement cost). This should work out better for players, as it plays against the
default of vision costs equaling movement costs.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56355 75b3775d-3801-0410-9e12-e2e43121521d