[2722438] Fixing crash when viewing shop contents. shop_item* was being blindly casted to monsters*, which didn't play well with a non-existent virtual function table. Fixed by creating a ShopItemEntry and a MonstersMenuEntry class to differentiate the meaning of MenuEntry::data and to avoid crashes in the future by having MenuEntry::get_tiles() be a no-op.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9572 c06c8d41-db1a-0410-9941-cceddc491573

Created by  ennewalker  on March 30, 2009
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