Also add variety to fixed level randarts, distinguishing between "easy"

(1-3), "moderate" (4-6) and "difficult" (7-9).

Recount how often each spell school is used and use the two most prominent schools (biasing by their order of definition in spl-util.h if two are tied). This doesn't completely work yet: I got a book of Enchantment and Divination although not a single spell in the book belonged to Divination (5 of them were Enchantment, so that was okay). Also, the way spells are currently selected (from two random disciplines) you can get a spell that does not fit either leading school (as later derived by counting) and might look odd with the title, e.g. a book is built on the premise of Conj/Fire and ends up as Conj/Air with Fire/Trm thrown in.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7848 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on December 16, 2008
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