Add a generic contains() function that detects if a container contains a given value.

This will use std::find in most cases, but uses the member find() function when
detecting if an associative container contains a key_value. So it should be
efficient, while allowing more readable code.

This function is in the util:: namespace to avoid any future naming conflicts.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56188 75b3775d-3801-0410-9e12-e2e43121521d