addon/mg: Use a multiset instead of a set to sort add-ons by title

This allows add-ons with identical titles to appear as many times as
necessary in the add-ons list instead of the first one clobbering the
rest.

Patch #3769 by thunderstruck. Fixes bug #20518.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56454 75b3775d-3801-0410-9e12-e2e43121521d