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/Print.hpp

@@ -444,7 +444,7 @@ public:
     bool                has_brim() const;
 
     // Returns an empty string if valid, otherwise returns an error message.
-    std::string         validate() const override;
+    std::string         validate(std::string* warning = nullptr) const override;
     double              skirt_first_layer_height() const;
     Flow                brim_flow() const;
     Flow                skirt_flow() const;