Removed per portal type filtering (yet keep compatibility with existing methods). Added a cache key so that filter expression results can be cached (ex. per portal_type). The general idea is that nearly nobody uses portal type filtering and that increasing the number of SQL methods tends to decrease performance.  By caching expression results, the impact of additional SQL methods is less significant. 
In addition, code syntax has been improved here and there (spaces, tabs). Some comments were added. Hardcoded values removed.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@17630 20353a03-c40f-0410-a6d1-a30d3c3de9de