Loosen the restriction regarding operator overloads by applying a (modified) patch from JC Chu. This fixes Mantis #22359.

The changes are built up in a way that all operators on two given types are not allowed if a default implementation for that operator exists. This implies that there is a possibility to have a overload work for (in this order) type A and B, but not for B and A. This is for example the case for Set + Enum (which seems to have a default implementation) where Enum + Set is allowed. The added tests try to detect as many default implementations as feasible, but can't cover everything...

git-svn-id: trunk@21975 -