Generate gizmos' tooltip on demand

src/slic3r/GUI/Gizmos/GLGizmoMove.hpp

@@ -30,6 +30,8 @@ public:
 
     const Vec3d& get_displacement() const { return m_displacement; }
 
+    virtual std::string get_tooltip() const;
+
 protected:
     virtual bool on_init();
     virtual std::string on_get_name() const;