Add a description to sewers (David) and replace the "melded" tile with

a grayish overlay, as suggested by Enne. The yellow background is now free to be used by something else (main weapon, maybe, if that can be derived somehow).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7587 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on November 24, 2008
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