#2 Allow multiple "FromItem" elements in "from" clause

[Raskolnikov]

In most databases, a join query can also be written as a "where" condition referring to multiple "from" items. (example: select t1.a, t2.b from t1, t2 where t1.id = t2.id). This is currently not supported since only one FromItem can be inspected (FromItem PlainSelect.getFromItem()). It would increase the usefulness if multiple "from" items would be allowed (List PlainSelect.getFromItemList()).