Add a notification when custom support enforcers are not used due to supports being off

It is now emitted from Print::validate and has a hyperlink to enable supports

src/libslic3r/SLAPrint.hpp

@@ -458,7 +458,7 @@ public:
 
     const SLAPrintStatistics&   print_statistics() const { return m_print_statistics; }
 
-    std::string validate() const override;
+    std::string validate(std::string* warning = nullptr) const override;
 
     // An aggregation of SliceRecord-s from all the print objects for each
     // occupied layer. Slice record levels dont have to match exactly.