Generate gizmos' tooltip on demand

src/slic3r/GUI/Gizmos/GLGizmoCut.hpp

@@ -28,6 +28,8 @@ public:
     double get_cut_z() const { return m_cut_z; }
     void set_cut_z(double cut_z) const;
 
+    virtual std::string get_tooltip() const;
+
 protected:
     virtual bool on_init();
     virtual void on_load(cereal::BinaryInputArchive& ar) { ar(m_cut_z, m_keep_upper, m_keep_lower, m_rotate_lower); }