Use mixin.CachedConvertableMixin. getConvertedSize removed from Document class (redundant with getConversionSize). docstring put again where it belongs (ie. Document class). Preparing the removal of document interface.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@29844 20353a03-c40f-0410-a6d1-a30d3c3de9de